Objective: To clarify the role of peritoneography in assessing the patency of processus vaginalis (PV) in pediatric patients diagnosed with cryptorchidism. Materials and Methods:We designed a prospective clinical trial to evaluate the patency of PV in boys presenting cryptorchidism. Herniography was performed in 310 prepubertal boys. Data about the morphology of PV was compared with operative findings in those surgically treated patients. Retractile and ectopic testes were excluded from the study. Results: Of the 376 undescended testes (310 patients), 281 cases were associated with an obliterated PV. Herniography revealed 95 cases of open PV in cryptorchid boys. The 244 normally descended testes had associated patent processus vaginalis in only 31 cases. Conclusions: Herniography is the most relevant procedure for accurate diagnosis of persistent PV. The persistence of PV was significantly more frequent when the position of the testes is more cranial. The incidence of an open PV decreases with age.