diff --git a/src/vs/base/parts/quickopen/browser/quickOpenViewer.ts b/src/vs/base/parts/quickopen/browser/quickOpenViewer.ts
index 9d0d465b63b1de9dcfa8710d18551f5ea0a95eb9..07817947cbb0d6bc9b0ca3f9c14c0a4a1d4face6 100644
--- a/src/vs/base/parts/quickopen/browser/quickOpenViewer.ts
+++ b/src/vs/base/parts/quickopen/browser/quickOpenViewer.ts
@@ -55,6 +55,16 @@ export class AccessibilityProvider implements IAccessibilityProvider {
 
 		return model.accessibilityProvider && model.accessibilityProvider.getAriaLabel(element);
 	}
+
+	public getPosInSet(tree: ITree, element: any): string {
+		const model = this.modelProvider.getModel();
+		return String(model.entries.indexOf(element) + 1);
+	}
+
+	public getSetSize(): string {
+		const model = this.modelProvider.getModel();
+		return String(model.entries.length);
+	}
 }
 
 export class Filter implements IFilter {
diff --git a/src/vs/base/parts/tree/browser/tree.ts b/src/vs/base/parts/tree/browser/tree.ts
index 30435a9ddb070e17c218feba604672f849416bba..4206026b17922cb7adf8bca4a2ba76a2fa224e87 100644
--- a/src/vs/base/parts/tree/browser/tree.ts
+++ b/src/vs/base/parts/tree/browser/tree.ts
@@ -428,6 +428,10 @@ export interface IAccessibilityProvider {
 	 * See also: https://www.w3.org/TR/wai-aria/states_and_properties#aria-label
 	 */
 	getAriaLabel(tree: ITree, element: any): string;
+
+	getPosInSet?(tree: ITree, element: any): string;
+
+	getSetSize?(): string;
 }
 
 export /* abstract */ class ContextMenuEvent {
diff --git a/src/vs/base/parts/tree/browser/treeView.ts b/src/vs/base/parts/tree/browser/treeView.ts
index 659c5360b7d53e03514f4d787db0830adb1313cb..02a5ce62343793bae67bbc228ab3914bc1bcb076 100644
--- a/src/vs/base/parts/tree/browser/treeView.ts
+++ b/src/vs/base/parts/tree/browser/treeView.ts
@@ -197,10 +197,15 @@ export class ViewItem implements IViewItem {
 
 		// ARIA
 		this.element.setAttribute('role', 'treeitem');
-		const ariaLabel = this.context.accessibilityProvider.getAriaLabel(this.context.tree, this.model.getElement());
+		const accessibility = this.context.accessibilityProvider;
+		const ariaLabel = accessibility.getAriaLabel(this.context.tree, this.model.getElement());
 		if (ariaLabel) {
 			this.element.setAttribute('aria-label', ariaLabel);
 		}
+		if (accessibility.getPosInSet && accessibility.getSetSize) {
+			this.element.setAttribute('aria-setsize', accessibility.getSetSize());
+			this.element.setAttribute('aria-posinset', accessibility.getPosInSet(this.context.tree, this.model.getElement()));
+		}
 		if (this.model.hasTrait('focused')) {
 			const base64Id = strings.safeBtoa(this.model.id);