One hundred twenty-eight prepubertal boys, ages 1-12 years, were referred to our institute because of uni- or bilateral cryptorchidism. Patients with mobile (retractile) testes were not included in the study. They were treated with human chorionic gonadotrophin (hCG, Pregnyl-Teva) twice weekly for five consecutive weeks by the following doses; 1-4 years (n = 53) - 250 U X 10; 4-7 years (n = 51) - 500 U X 10; 7-12 years (n = 24) - 1000 U X 10;i.m. Plasma testosterone (T) levels were measured before and once weekly during the treatment period. A good clinical response (a complete descent of the testes into the scrotum) was obtained in 32 (25%), a partial response (a distal movement of the testes towards the scrotum) - in 41 (32%) of the boys. The basal plasma T levels were within the normal range for prepuberty in all the patients (18 +/- 9 ng/dl) and a significant increase during hCG treatment was found in all (peak of 344 +/- 185 to 640 +/- 288 ng/dl). These values were not significantly different between the "responders", "partial responders". The result of these studies, indicate the use of hCG for the initial treatment of boys with cryptorchidism, the minimal effective dose being 500 U twice weekly for five consecutive weeks.