diff --git a/src/main/java/it/inaf/ia2/transfer/controller/PutFileController.java b/src/main/java/it/inaf/ia2/transfer/controller/PutFileController.java
index 440f8a3e1e9a0867e3032db9fdd0654488634515..0314f7b9a08d7ad2caef13b26ff11db6d1d420de 100644
--- a/src/main/java/it/inaf/ia2/transfer/controller/PutFileController.java
+++ b/src/main/java/it/inaf/ia2/transfer/controller/PutFileController.java
@@ -83,9 +83,15 @@ public class PutFileController {
 
         File file = path.toFile();
 
-        if (!file.getParentFile().exists()) {
-            if (!file.getParentFile().mkdirs()) {
-                throw new IllegalStateException("Unable to create parent folder: " + file.getParentFile().getAbsolutePath());
+        /**
+         * This block must be synchronized, to avoid concurrency issues when
+         * multiple files are uploaded to a new folder in parallel.
+         */
+        synchronized (this) {
+            if (!file.getParentFile().exists()) {
+                if (!file.getParentFile().mkdirs()) {
+                    throw new IllegalStateException("Unable to create parent folder: " + file.getParentFile().getAbsolutePath());
+                }
             }
         }