diff --git a/src/diagnosis.ts b/src/diagnosis.ts
new file mode 100644
index 0000000..0da80e2
--- /dev/null
+++ b/src/diagnosis.ts
@@ -0,0 +1,53 @@
+/**
+ * diagnosis.ts
+ * 
+ * @author CismonX <admin@cismon.net>
+ * @license MIT
+ */
+
+import * as vscode from 'vscode';
+import { isDefined, lineNumToRange } from './utils';
+
+/**
+ * Manage diagnostic information of Texinfo documents.
+ */
+export default class Diagnosis implements vscode.Disposable {
+
+    private static singleton?: Diagnosis;
+
+    static get instance() {
+        return Diagnosis.singleton ??= new Diagnosis();
+    }
+
+    private readonly diagnostics = vscode.languages.createDiagnosticCollection('texinfo');
+
+    dispose() {
+        this.diagnostics.dispose();
+    }
+
+    /**
+     * Generate diagnostic information based on error log from `makeinfo`.
+     * 
+     * @param document 
+     * @param logText 
+     */
+    update(document: vscode.TextDocument, logText: string) {
+        const fileName = document.uri.path;
+        const diagnostics = logText.split('\n').filter(line => line.startsWith(fileName))
+            .map(line => logLineToDiagnostic(line.substring(fileName.length + 1))).filter(isDefined);
+        this.diagnostics.set(document.uri, diagnostics);
+    }
+
+    delete(document: vscode.TextDocument) {
+        this.diagnostics.delete(document.uri);
+    }
+}
+
+function logLineToDiagnostic(lineText: string) {
+    const lineNum = Number.parseInt(lineText) - 1;
+    // Ignore error that does not correspond a line.
+    if (Number.isNaN(lineNum)) return undefined;
+    const message = lineText.substring(lineNum.toString().length + 2);
+    const severity = message.startsWith('warning:') ? vscode.DiagnosticSeverity.Warning : undefined;
+    return new vscode.Diagnostic(lineNumToRange(lineNum), message, severity);
+}
diff --git a/src/extension.ts b/src/extension.ts
index e390343..bbe8d8b 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -6,6 +6,7 @@
  */
 
 import * as vscode from 'vscode';
+import Diagnosis from './diagnosis';
 import Document from './document';
 import Logger from './logger';
 import Options from './options';
@@ -25,6 +26,7 @@ export function activate(context: vscode.ExtensionContext) {
         vscode.languages.registerCompletionItemProvider('texinfo', new CompletionItemProvider(), '@'),
         vscode.languages.registerFoldingRangeProvider('texinfo', new FoldingRangeProvider()),
         vscode.languages.registerDocumentSymbolProvider('texinfo', new DocumentSymbolProvider()),
+        Diagnosis.instance,
     );
 }
 
diff --git a/src/preview.ts b/src/preview.ts
index 82f38ed..695d8c0 100644
--- a/src/preview.ts
+++ b/src/preview.ts
@@ -8,6 +8,7 @@
 import * as path from 'path';
 import * as vscode from 'vscode';
 import Converter from './converter';
+import Diagnosis from './diagnosis';
 import Document from './document';
 import Logger from './logger';
 import Options from './options';
@@ -70,6 +71,8 @@ export default class Preview {
         this.disposables.forEach(event => event.dispose());
         this.panel.dispose();
         this.documentContext.closePreview();
+        // Only show diagnostic information when the preview is active.
+        Diagnosis.instance.delete(this.document);
     }
 
     async updateWebview() {
@@ -81,20 +84,23 @@ export default class Preview {
         this.pendingUpdate = false;
         // Inform the user that the preview is updating if `makeinfo` takes too long.
         setTimeout(() => this.updating && this.updateTitle(), 500);
-        let htmlCode = await Converter.convertToHtml(this.document.fileName);
-        if (htmlCode === undefined) {
+        const { data, error } = await Converter.convertToHtml(this.document.fileName);
+        if (error) {
+            Logger.instance.log(error);
+            Diagnosis.instance.update(this.document, error);
+        }
+        if (data === undefined) {
             prompt(`Failed to show preview for ${this.document.fileName}.`, 'Show log', true)
-                .then(result => result && Logger.show());
+                .then(result => result && Logger.instance.show());
+        } else if (Options.displayImage) {
+            const pathName = path.dirname(this.document.fileName);
+            // To display images in webviews, image URIs in HTML should be converted to VSCode-recognizable ones.
+            this.panel.webview.html = transformHtmlImageUri(data, src => {
+                const srcUri = vscode.Uri.file(pathName + '/' + src);
+                return this.panel.webview.asWebviewUri(srcUri).toString();
+            });
         } else {
-            if (Options.displayImage) {
-                const pathName = path.dirname(this.document.fileName);
-                // To display images in webviews, image URIs in HTML should be converted to VSCode-recognizable ones.
-                htmlCode = transformHtmlImageUri(htmlCode, src => {
-                    const srcUri = vscode.Uri.file(pathName + '/' + src);
-                    return this.panel.webview.asWebviewUri(srcUri).toString();
-                });
-            }
-            this.panel.webview.html = htmlCode;
+            this.panel.webview.html = data;
         }
         this.updating = false;
         this.updateTitle();
diff --git a/src/symbol.ts b/src/symbol.ts
index aa6e124..7bcada4 100644
--- a/src/symbol.ts
+++ b/src/symbol.ts
@@ -8,7 +8,7 @@
 import * as vscode from 'vscode';
 import Document from './document';
 import { FoldingRange } from './folding';
-import { Optional } from './utils';
+import { lineNumToRange, Optional } from './utils';
 
 /**
  * Provide document symbol information for Texinfo documents.
@@ -60,11 +60,8 @@ function foldingRangeToSymbols(ranges: readonly RangeNode[], start: number, end:
     for (let idx = start; idx < end; ++idx) {
         const node = ranges[idx];
         if (node === undefined) continue;
-        const startPosition = new vscode.Position(idx, 0);
-        const endFirstLine = new vscode.Position(idx, Number.MAX_SAFE_INTEGER);
-        const endLastLine = new vscode.Position(node.end, Number.MAX_SAFE_INTEGER);
-        const range = new vscode.Range(startPosition, endLastLine);
-        const selectionRange = new vscode.Range(startPosition, endFirstLine);
+        const range = lineNumToRange(idx, node.end);
+        const selectionRange = lineNumToRange(idx);
         const symbol = new vscode.DocumentSymbol('@' + node.name, node.detail,
             vscode.SymbolKind.String, range, selectionRange);
         symbol.children = foldingRangeToSymbols(ranges, idx + 1, node.end);
diff --git a/src/utils.ts b/src/utils.ts
index 60d0c1d..647898e 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -60,6 +60,22 @@ export function transformHtmlImageUri(htmlCode: string, transformer: (src: strin
     return elements.length === 0 ? htmlCode : dom.outerHTML;
 }
 
+/**
+ * Convert line numbers to VSCode range.
+ * 
+ * @param startLine 
+ * @param endLine Default to `startLine`.
+ */
+export function lineNumToRange(startLine: number, endLine = startLine) {
+    const startPosition = new vscode.Position(startLine, 0);
+    const endPosition = new vscode.Position(endLine, Number.MAX_SAFE_INTEGER);
+    return new vscode.Range(startPosition, endPosition);
+}
+
+export function isDefined<T>(value: T | undefined): value is T {
+    return value !== undefined;
+}
+
 export type Optional<T> = T | undefined;
 
 export type ExecResult = { data?: string, error: string };