The sonographic findings of 125 renal masses that did not meet the sonographic criteria of benign simple cyst were retrospectively reviewed and correlated with the pathologic diagnosis. Of the 125 masses, 102 were renal cell carcinoma. These carcinomas, two of nine angiomyolipomas, and the 14 other renal masses of various histologic types exhibited a broad spectrum of sonographic findings. Seven of nine angiomyolipomas and the one multiloculated cystic nephroma had a rather characteristic sonographic appearance. With the knowledge of this spectrum of sonographic findings and strict attention to scanning techniques and sonographic criteria, the radiologist should be able to define the characteristics of a variety of renal masses and suggest the correct diagnosis. It should be possible to make the diagnosis of angiomyolipoma confidently if the characteristic sonographic and computed tomographic appearance of these tumors can be demonstrated. In some cases, the correct histologic diagnosis of a renal mass can be made only with biopsy or surgery.