The importance of venous distension of the pampiniform plexus in the prevention of inguinal hernia was evaluated. Functional closure of the inguinal canal was investigated by computed tomography during a Valsalva manoeuvre in a normal individual and 12 men with unilateral inguinal herniation, one of whom had previously undergone herniorrhaphy. Thus, 13 normal canals were evaluated along with five with direct and seven with indirect hernia. In all normal canals a rise in intra-abdominal pressure increased the cross-sectional area of the part of the pampiniform plexus located in the medial end of the canal, thereby filling the empty space and preventing herniation. In the lateral part the plexus was compressed against the external aponeurosis by the fascia transversalis in a valve-like fashion. In cases of direct inguinal herniation the distended pampiniform plexus was displaced by the hernia sac during the Valsalva manoeuvre, but in indirect hernia impaired swelling of the pampiniform plexus was seen. It is suggested that this observation could help to explain the normal mechanism of closure of the inguinal canal.