CT diagnosis of renal lesions proved to be highly efficient and reliable. This modality was most accurate in diagnosing renal cysts and the great majority of solid lesions. There was an indeterminate group (5% of cases studied) that included cysts, tumors and abscesses. This group requires additional investigation, but angiography is often not conclusive because of the avascular nature of these tumors. Other problem areas included cysts with apparently irregular margins and artifacts produced by the presence of contrast agent in the collecting systems.