diff --git a/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/ExternalFoldersManager.java b/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/ExternalFoldersManager.java
index d615698..192d097 100644
--- a/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/ExternalFoldersManager.java
+++ b/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/ExternalFoldersManager.java
@@ -122,7 +122,7 @@
 		} while (result.exists());
 		if (scheduleForCreation) {
 			if (this.pendingFolders == null)
-				this.pendingFolders = new HashSet();
+				this.pendingFolders = Collections.synchronizedSet(new HashSet());
 			this.pendingFolders.add(externalFolderPath);
 		}
 		knownFolders.put(externalFolderPath, result);
@@ -166,13 +166,15 @@
 		catch(CoreException e) {
 			throw new JavaModelException(e);
 		}
-		Iterator iterator = this.pendingFolders.iterator();
-		while (iterator.hasNext()) {
-			Object folderPath = iterator.next();
-			try {
-				createLinkFolder((IPath) folderPath, false, externalFoldersProject, monitor);
-			} catch (CoreException e) {
-				Util.log(e, "Error while creating a link for external folder :" + folderPath); //$NON-NLS-1$
+		synchronized (this.pendingFolders) {
+			Iterator iterator = this.pendingFolders.iterator();
+			while (iterator.hasNext()) {
+				Object folderPath = iterator.next();
+				try {
+					createLinkFolder((IPath) folderPath, false, externalFoldersProject, monitor);
+				} catch (CoreException e) {
+					Util.log(e, "Error while creating a link for external folder :" + folderPath); //$NON-NLS-1$
+				}
 			}
 		}
 		this.pendingFolders.clear();