The epididymal anomalies and patent processus vaginalis are frequently seen in patients with cryptorchidism or hydrocele. We performed a prospective study on the relationship between the epididymal anomalies and the patency of the processus vaginalis in boys with hydrocele (190 cases) or cryptorchidism (89 cases) who were treated from August 1997 to February 2000 (mean age, 51 months; range, 12 to 152 months). The epididymal anomalies were observed with an overall frequency of 48%. Closed, partially closed, and open processus vaginalis were associated with an epididymal anomaly in 14, 38, and 65% of cases, respectively. The epididymal anomalies were more common in association with undescended (61%) than with descended (43%) testes without statistical significance (p=0.415). Incomplete attachment of the caput epididymis was the most common anomaly (35%), followed by detachment of caput and cauda epididymis (31%), cauda epididymis (24%), and long looping epididymis (10%). These data showed that the epididymal anomalies were strongly associated with the patency of the processus vaginalis irrespective of testicular descent (p<0.001), and they provide further evidence for the hypothesis that a common stimulus, possibly androgens, may be required for the epididymal development and obliteration of the processus vaginalis.