Preventable mishaps in an intensive therapy unit were studied over 12 months by the critical incident technique. Staff were encouraged to complete confidential questionnaires describing incidents in which they had participated or had observed. This allowed classification of the events and examination of the views of staff on causes, detection, and prevention. 110 (80%) of 137 events were felt to have been due to human error; the remainder were due to equipment failure. Inexperience with equipment and shortage of trained staff were the factors most often felt to contribute to incidents. The critical incident technique is a useful way of improving standards of clinical care.