Sonography was utilized to assess six patients with renal transitional cell carcinoma and was correlated with gross pathology in five cases. The neoplasms were seen as discrete, solid masses causing separation of the central renal sinus echoes. The echogenicity of the tumors and the renal parenchyma was similar, but the neoplasms were separated from the renal parenchyma by a linear echogenic zone of renal sinus fat. Pelvicaliceal blood clots, when markedly echogenic, could be differentiated from transitional cell carcinomas. Sonography contributed most in evaluating unilateral nonvisualized kidney or in the presence of nonfilling of the affected pelvicalices on excretory urography.