Computed tomography (CT) is the current standard for diagnosing and staging renal cell carcinoma (RCC). Although general diagnostic guidelines exist, no large studies to date have delineated the CT features of RCC. We reviewed the CT appearances of 78 pathologically proven RCCs. Of the 61 RCCs larger than 50 mm (78%) there was imaging evidence of extrarenal spread (87%), intratumoral necrosis (61%) and differential growth rates within the tumor (64%). Tumors 50 mm or smaller often had a "benign" appearance with sharp, rounded margins (88%), homogeneous density (65%), and distinct interface with the kidney (82%). The significance of these lesions should not be underestimated. Although RCCs often showed transient marked enhancement after bolus contrast material injection (41%), during the infusion phase 97% were hypodense compared with the kidney regardless of tumor size. Calcifications were visible in 31% of RCCs. Although 22% of RCCs were predominantly cystic, none fulfilled all CT criteria of simple renal cysts.