Objective-To design and test a simple scale for measuring the severity of diagnostic errors occurring in accident and emergency (A&E) departments. Methods-Empirical design of a scale which indicates the severity of errors on a scale of 1 to 7. It is obtained by adding two scores which indicate the additional treatment which a patient would have received and the follow up which would have been organised if the correct diagnosis had been made initially. Results-The misdiagnosis severity score (MSS) revealed 166 diagnostic errors in injuries treated in an A&E department over one year. The scoring system allowed the more significant errors to be separated from the less significant ones. Conclusions-The MSS proved useful in describing the errors made in an A&E department. (X Accid Emerg Med 1997;14:290-292)