In an attempt to define what factors are important to the development of postanaesthetic complications, the data from a nine-year prospective study of anaesthetic practice in a large tertiary care institution were evaluated. A model of anaesthetic morbidity dependent upon factors of patient illness, surgical practice, anaesthetic technique and physician experience, and duration of anaesthesia was developed. Postoperative anaesthetic morbidity was defined as any anaesthetic-related complication which, in the opinion of the follow-up nurse, was associated with prolonged hospitalization or documented compromise of the patient. Using a multiple logistic regression, ASA physical status was a risk factor for postoperative complications (odds ratio = 1.95) but the number of preoperative conditions and age were not. The type of surgical procedure, classified by site or by degree of trauma, did not influence postanaesthetic complication rates. The duration of anaesthetic exposure was an important determinant of risk (odds ratio = 2.53), with complications increasing with the length of anaesthetic time. As to factors under control of the anaesthetist those patients experiencing operating room complications (odds ratio = 3.36) or those receiving pure spinal (odds ratio = 5.53) or narcotic techniques (odds ratio = 2.14) had higher risks of postoperative complications. Finally, it would appear that the greater the experience of the anaesthetist the less the risk of postoperative problems (odds ratio = 0.52).