A 22 year old XX female patient with primary amenorrhoea and sexual infantilism was studied. Persistently elevated serum LH and FSH concentrations and exaggerated LRH pituitary responsiveness indicated deficient ovarian hormonal production. Serum levels of C21 and C19 steroids measured by specific radioimmunoassays before and after appropriate stimulations demonstrated an impairment of adrenal and ovarian steroid biosynthesis. Baseline levels of androstenedione (delta 4-A), testosterone (T), and oestradiol-17 beta (E2) were persistently below the normal range for healthy women at early follicular phase, whereas progesterone (P) and 17 alpha-OH-progesterone (17-OH-P) serum levels were significantly higher than those observed for normal women. Adrenal and gonadal stimulation with ATCH and hCG, respectively, resulted in a considerable rise in serum P and 17-OH-P without any significant change in circulating levels of delta 4-A, T, and E2. These findings were consistent with the diagnosis of 17,20 steroid desmolase deficiency at both adrenal and ovarian levels. This is the first report of a 17,20 desmolase deficiency in an XX individual, and is in line with previous suggestions that familial occurrence of the disorder would fit an autosomal recessive pattern of inheritance.