Background and objectives.
ABO-incompatible red blood cell (RBC) transfusions are a major risk in transfusion medicine. Identification of factors leading to this hazard is important to improve transfusion safety.

Material and methods.
All consecutive erroneous ABO-incompatible transfusions occurring from January 1997 to December 2004 at the Charité University Hospital in Berlin, Germany were analysed.

Results.
A total of 343,432 RBC units were transfused, and eight patients erroneously received 13 ABO-incompatible RBC concentrates. The most frequent error was incorrect bedside testing (n = 7). Intensive care treatment was required in two cases, but there were no fatal mistransfusions. Four patients had no or only mild reactions.

Conclusion.
Mistransfusions are still a considerable risk in transfusion medicine despite quality control systems and electronic data processing. An increase in transfusion safety may require the introduction of further systems, e.g. radio-frequency identification (RFID) tags.