Most serious transfusion error is ABO-incompatible transfusion. The Japanese Society of Blood Transfusion reported the results of the National Survey on the present state of ABO-incompatible blood transfusion in Japan. The targets of the survey were 777 hospitals with more than 300 beds, which have transfused more than 3,000 units of blood products per year. The answer could be obtained from 578(74%) hospitals among them. The questionnaire focused on the presence or absence of ABO-mismatched transfusion between January 1995 and December 1999. 115 hospitals(20%) have experienced ABO-mismatched transfusion in the period of the study. The frequency of ABO-mismatched transfusion increased with increasing the number of beds and the units of transfused blood. The ABO-mismatched transfusions were due to clerical or technical errors. The main causes of errors were misidentification of blood bags(42.8%), incorrect blood typing(15.1%) and failure to identify patient (11.5%). Incorrect blood typing errors were performed in 25 cases, and were caused by doctors in 17 cases, others by laboratory technicians. Majority of incorrect blood typing occurred during holidays or night shifts, and in emergency.