Problem is that other calls to windows API's overwrite the error code. I fund that the following helped 1. Before calling any Windows APIs that set the errorcode call GetLastError, this ensires that jna has aet up the call. 2. Make sure that you retriere the error code immediately after the suspect call. 3. Running the JVM in debug mode always seems to overwrite the errorcode. However even when I had followed all these guidelines I still find that occasionally the errorcode is lost. This occurs in code within a for loop, so the results are not detrministic. I think that under some circumstances the JVM does extra windows calls on the same native thread , but I dont know how to trap or detect them.