CT [computed tomography] diagnosis of renal lesions was highly efficient and reliable [in humans]. 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 required additional investigation, but angiography was 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.