Within a 24-month period, 5 patients in a large teaching hospital were mistakenly transfused with blood that had been crossmatched for different patients. Each of the incidents was due to failure by ward staff to adhere to established safely procedures. Three incidents were entirely due to failure to make the standard checks comparing the identity details on the blood pack label with the patient identification details. The fourth incident was due to a combination of inadequate checking and incomplete patient identification procedure. The fifth incident was due to a series of errors that consisted of inadequate checking, plus putting the wrong patient's blood into the sample tube, plus misspelling of the patient's name on both sample tube and request form. There was no serious morbidity. All of these incidents took place when an unusual coincidence or contributing error lead to unmasked inadequate checking of blood unit against patient's identity by ward staff. The incidence of inadequate checking technique may be much higher than the incidence of erroneous transfusion events. It may be difficult to test the ability of an 'improved' transfusion procedure to prevent disasters from erroneous unit-to-patient matching, since assessment will need to include actual 'worst case' situations, which should be rare.