Epidemiologic studies of diagnostic error in the intensive care unit (ICU) consist mostly of descriptive autopsy series. In these studies, rates of diagnostic errors are approximately 5% to 10%. Recently validated methods for retrospectively measuring error have expanded our understanding of the scope of the problem. These alternative measurement strategies have yielded similar estimates for the frequency of diagnostic error in the ICU. Although there is a fair understanding of the frequency of errors, further research is needed to better define the risk factors for diagnostic error in the ICU.