Concurrent programming is a complex task, even with modern languages such as Java who provide language-based support for multithreading and synchronization. In addition to typical errors from sequential programming concurrent programming is prone to security and lifeness errors, which are difficult to detect due to the inherent nondeterminism in concurrent programs. While debugging is still mainly based on textual representations, we think that the use of visual languages can ease program comprehension. Once a synchronization error is detected, e.g. during testing, the error situation shall be visualized to analyze the reason for the error. With UML being a major visual modeling language for object oriented software development we decide to base our visualization on it and present how to visualize program traces with UML sequence and collaboration diagrams. We focus on the visualization of the synchronization of threads. For this purpose we extend UML to model the runtime mechanisms of the Java language constructs for synchronization.