A new case of familial idiopathic hypogonadotrophic hypogonadism is presented: 3 males and 2 females among 9 siblings are affected. Very low to non-detectable levels of plasma LH and FSH levels were found in each patient. LH-RH stimulation test gave in one male a slight increase in plasma LH levels while no change was observed in the others. Prolonged treatment with gonadotrophins or testosterone gave poor results in the males while evidence of ovulation was obtained in the two females during a unique induced artificial cycle with combined HMG and HCG treatment; rapid feminization was also obtained in the females with oestrogen therapy. The striking difference in the results of the substitution treatments between males and females suggest that some degree of acquired insensitivity of the testes to gonadotrophins and of peripheral tissues to male sex hormones are present in male hypogonadotrophic hypogonadism. This may be the consequence of a lack of hormonal stimulation or impregnation during infancy and childhood