One of the most complex and challenging tasks facing physicians is the need to make a diagnosis. Recent research has focused attention on medical errors alleged to have resulted in increased patient morbidity and mortality. A number of authorities have suggested methods to track and prevent errors. Most have dealt with systems' changes and fail-safe methods to prevent medication errors and commission of errors during treatment. Few have addressed methods to find and prevent diagnostic errors. Unless diagnostic error is either avoided or corrected early, fail-safe methods to prevent medication and treatment errors will ultimately fail to improve patient outcome. American medical literature, particularly postmortem studies, have documented diagnostic error since at least 1912. European literature shows the problem is worldwide. The limits of human memory and errors in both observation and processing of information during problem solving contribute to the commission of errors. The purpose of this article is to examine the thinking patterns and cognitive errors that can result in diagnostic error, and suggest instructional strategies that can be used to alert residents and attending physicians to these potential problems so they can be avoided.