PURPOSE: This study was aimed to review findings of gray-scale ultrasonography and to assess the diagnostic value of the resistive index (RI) in patients with acute epididymitis by comparing with that in normal volunteers. MATERIALS & METHODS: Gray-scale ultrasonogram was reviewed in nine patients with acute epididymitis for echogenicity, size and reactive hydrocele. Normal values of resistive index (RI) were obtained in 20 epididymides from 10 volunteers (aged 20-28, mean 23.7). Nine patients of epididymitis (aged 18-67, mean 37.2) were examinated with color Doppler sonography and RI was measured at 11 sites on epididymal arteries. For the prediction of acute epididymitis, accuracy, sensitivity and specificity were calculated at the cut-off value of RI in 0.65 and 0.70. RESULTS: Gray-scale ultrasonography showed echogenicity that was variable among the 9 cases (hype- rechoic 2 cases, isoechoic 2, hypoechoic 5), enlargement of epididymis in 8, and reactive hydrocele in 8 cases. Normal volunteers displayed color Doppler signals in 14 out of 20. The range of RI in normal volunteers were between 0.64 and 1.00 (mean, 0.79 +/- SD 0.10). In patients with acute epididymitis, color Doppler signals were detected in all patients. The range of RI in epididymitis were 0.40-0.68 (mean, 0.56 +/- SD 0.10). At cut-off RI value of 0.65, accuracy was 88%, sensitivity 81.8%, and specificity 92.7%. At cut-off value of 0.70, accuracy was 92%, sensitivity 100%, and specificity 85.7%. CONCLUSION: In addition to the findings of gray-scale ultrasonography, resistive index and color flow changes are valuable in diagnosing acute epididymitis.