57 patients were studied to determine the validity of post-voiding films in routine urography. It was discovered that if the post-voiding film was delayed by as much as five minutes after the patient had actually voided, a significant error was introduced. The post-voiding film must therefore be obtained immediately after voiding and even a short delay, which is not uncommon in busy Radiology Ddepartments, may result in a false impression of incomplete voiding.