Temperature of the undescended testis, measured in its cryptorchid location during surgical procedure for orchidopexy in 46 boys, 13- to 180-months-old, was significantly higher (34.4 +/- 0.9 degrees C) than that of the contralateral normally descended testicle (33.2 +/- 1.2 degrees C; P < 0.001). Temperature significantly declined from the Bogros' space (35.3 +/- 0.5 degrees C) to the inguinal canal (34.7 +/- 0.7 degrees C) and from the latter to the empty scrotal cavity (31.1 +/- 1.8 degrees C). The physiological descent of the testicle is associated with a marked cooling of its thermal environment. When this process is interrupted for any reason, temperature of the cryptorchid testis is significantly increased. This rise in temperature could be either a concomitant or a main factor for the impairement of testicular functions associated with cryptorchidism.