Real-time sonography of the scrotal veins was performed in 13 subjects with clinically obvious or small varicocele and in 10 normal controls. In normals, the vessels were 0.5-1.5 mm in caliber and a main draining vein up to 2 mm often was seen. In all varicoceles, numerous dilated, tortuous, branching vessels of uniform size were observed. Vessels of different lesions varied in caliber from 2 to 5 mm. Blood flow was seen in some normal vessels, and sluggish flow was observed in all varicoceles. The direction of visualized flow and the influence on flow of the upright position and Valsalva maneuver were shown with confidence only in large and medium-sized lesions. In these the findings were consistent with incompetence of the internal spermatic venous system. The cystic spaces in multiloculated spermatoceles or epididymal cysts varied more in size, were not tubular or branching, and no flow phenomenon was seen at high gain settings. Sonography provides an alternative to other noninvasive tests for detection of a small varicocele, especially in the infertile patient.