Five patients with renal cell carcinoma undiagnosed by excretory urogram were reported. Four of five patients were detected incidentally. Excretory urograms show normal appearance in three cases and retrospectively small protruding nodule from the lateral surface of the tumor bearing kidney in two cases. In all cases, hyperechoic mass was detected ultrasonographically. The size of tumor in four cases was less than 3.0 cm in diameter. Four cases were stage I and one case was stage II by Robson's cliteria. Ultrasonography is able to detect the small size of renal tumor which cannot be diagnosed by excretory urogram. Ultrasonography may be the most useful diagnostic procedure to screen the mass lesion of kidney.