The types and frequency of complications attributable solely to anaesthesia, and which caused serious morbidity and substantial negative outcome, were examined in a prospective study of 7306 anaesthetics. The analysis demonstrated that severe complications and mortality attributable to anaesthesia occurred in 0.59% (1:170) and 0.04% (1:2500) of patients respectively. The data suggest that "high-risk' patients are more likely to be affected by errors and a substantial negative outcome than more healthy patients. We judged one-third of the anaesthesia-related morbidity to be preventable. Anaesthetists are faced with an increasing number of patients who are at high risk, and who undergo increasingly extensive surgical interventions. Concomitant with the increasing complexity of care and increasing demands on the anaesthetists may be an expected increase in the incidence of errors. Recognition of the importance of human factors in morbidity and mortality attributable to anaesthesia is a necessary first step.