In a retrospective review of transfusion errors in a large teaching hospital, we found the true incidence of errors to be at least four times the actual mistransfusion events detected. Seventy-five per cent of the errors were detected as near misses. The mistransfusions equated to 1/8610 compatibility procedures, and 1/27 007 units of blood issued, whereas the number of true transfusion errors equates to 1/2153 compatibility procedures and 1/6752 units of blood issued. The major error-prone activities included patient identification at phlebotomy and the final infusion of the blood product at the bedside. Of the cases, 95.2% were due to non-compliance with existing guidelines. Potential disasters were avoided only by the vigilance of the blood bank staff and the systems in place to detect errors.