Five boys, each with at least 4 features characteristic of Noonan's syndrome, were studied throughout childhood. In each the height was on or below the 3rd centile. Height velocity was essentially normal during the prepubertal period whereas bone age and pubertal development were delayed. Four boys had cryptorchidism and in 2 no testicular enlargement or spontaneous puberty occurred, necessitating testosterone replacement. Plasma concentrations of luteinising hormone and follicle-stimulating hormone before and after stimulation with luteinising hormone-releasing hormone were increased in 4 of the boys. These findings are consistent with primary gonadal failure associated with impaired spermatogenesis.