A female patient with hypertension, hypogonadism and hypokalaemia due to 17alpha-hydroxylase deficiency is described, in whom the diagnosis was initially based on the finding of low urinary 17-ketosteroids and 17-hydroxycorticosteroids, in the presence of high plasma fluorometric 11-hydroxycorticoids. Later studies confirmed the diagnosis by demonstrating low circulating levels of 17alpha-hydroxyprogesterone and the steroids derived from this precursor: 11-deoxycortisol, cortisol, oestradiol and testosterone. Plasma progesterone, corticosterone and deoxycorticosterone (DOC) were greatly increased, but plasma and urine aldosterone were very low. The initial subnormal cortisol and 11-deoxycortisol levels responded to tetracosactrin administration, suggesting that the adrenal was not maximally stimulated by endogenous ACTH and indicating that the 17alpha-hydroxylase deficiency was incomplete. Plasma and urinary aldosterone levels were still suppressed after 12 months treatment with prednisone despite normalization of the previously suppressed renin levels. The findings indicate that this uncommon cause of medically-controllable juvenile hypertension can be identified without specific steroid assays.