Nine small renal masses with attenuation coefficients greater than 20 HU on computed tomography (CT) were termed indeterminate for cyst or renal cell carcinoma. Correlative sonography identified the fluid-filled internal architecture in four of five cysts and correctly characterized as solid four small renal cell carcinomas. The sonogram was particularly valuable in assessing masses that were indeterminate on CT when enhancement was equivocal or when intravenous contrast could not be administered. We conclude that sonography is extremely useful as a noninvasive procedure in evaluating the occasional small renal mass with CT number greater than 20 HU.