Cancellation of scheduled surgery is undesirable for patients and an inefficient use of resources. We prospectively collected data for 52 consecutive months in a public general hospital to estimate the prevalence and causes. The overall cancellation rate was 6.5% (2559 of 39 115 scheduled operations). Cancellation by broad category was for 'medical reasons' in 50%, 'patient-related factors' in 23%, and due to 'administrative/logistic problems' in 25%. The commonest specific causes within these categories were respectively: infections/fever (18%), patient did not attend (20%) and lack of theatre time (23%). This data will help direct resources to target prevention of cancellations as a result of these main problems.