Objective: To investigate the specific ultrasonographic changes of proximal deferent duct and diagnostic value of ultrasonography in patient with congenital absence of vas deferens (CAVD). Methods: Fifty eight cases with congenital bilateral absence of vas deferens (CBAVD) and 12 cases with congenital unilateral absence of vas deferens (CUAVD) were observed by high frequency scrotal ultrasonography, and the ultrasonographic features of proximal deferent duct were analyzed retrospectively. Results: All 128 proximal deferent ducts exhibited abnormal ultrasonography. In 42 (32.81%) cases only epididymal head was visible. Epididymal head along with partial body and tail were found in 48 (37.50% ) cases, in which 44 (34.38%) cases showed abrupt tapering and 4 (3.13%) showed cord-like changes. Vas deferens were not found in 31(24.22%) cases with whole process of epididymal head observed, body and tail. Whole epididymis along with partial vas deferens were observed in 7 (5.47%). Tubular ectasia of epididymis were found in CAVD, and the degree of dilation was greater than that of normal epididymis (P<0.01). The absence of epididymal tail-vas deferens ring accompanied with epididymal tubular ectasia was a characteristic change of CAVD. By taking the absence of epididymal tail-vas deferens ring into account, the sensitivity and specificity of scrotal US for diagnosing CAVD were 94.53% and 97.00%, respectively. Conclusions: High frequency scrotal ultrasound demonstrated well the structure of proximal deferent duct, and it is proved to be useful in evaluating the morphology, echo and characteristic dilatation of epididymis. It can provide valuable information for the clinical diagnosis of CAVD and judgment of the missing parts.