In a double-blind placebo-controlled study in 49 boys with cryptorchidism the effect of intranasal synthetic LHRH was studied. After 8 weeks improvement in testicular location was found in 13 testes (37%), but this improvement was considered sufficient of only six testes. Placebo resulted in an improved location in 18% of the testes. The mean change in testicular position (expressed in cm) after LHRH therapy was slightly greater than after placebo but only in the squatting position did this difference reach significance. Aggressive behaviour was reported in 23% of the children treated with LHRH. A second LHRH course did not result in significant improvement in any of the patients. At follow-up reascent was frequently seen. The final results in unilateral cryptorchidism are poorer than those in bilateral cryptorchidism. LHRH therapy leads to higher plasma LH levels and a lower FSH in response to an intravenous LHRH test. In 15 boys plasma testosterone levels rose above 0.4 nmol/l. We conclude that intranasal LHRH application has a limited value for the treatment of cryptorchidism but may be suitable as a diagnostic test.