Color flow Doppler sonography (CFDS) has been used to evaluate the blood flow to the testes, because of its ability to visualize small vessels with low flow. A total of 39 infertile men, 18 with clinical varicocele and 21 without, were examined using CFDS to assess the clinical usefulness of this technique. In patients with varicocele, venous diameter in the pampiniform plexus was greater than 2 mm, whereas those without varicocele showed diameters of less than 1.8 mm. Of 21 patients without varicocele on physical examination, 13 were found to have reflux by CFDS. According to clinical and CFDS findings, patients were divided into five groups: varicocele negative, suspicious subclinical varicocele, subclinical varicocele, manifest varicocele with Valsalva-induced reflux, and manifest varicocele. These data suggest that the CFDS technique is a simple, sensitive and noninvasive modality for the accurate diagnosis of varicocele.