A 51-year-old female was thought to have Conn's syndrome because she had hypertension, hypokalaemia and low plasma renin activity. The cause was not aldosterone excess, but there was an adrenal cortical carcinoma producing 11-deoxycorticosterone (DOC) in extremely large quantities, with ineffective 1 l/?-steroid hydroxylation. Plasma and urinary aldosterone levels were within the normal range. Blood levels of other steroids including those on the pathways of formation of glucocorticoids and sex steroids were unaffected. The tumour was cultured in vitro and shown by high performance liquid chromatography (HPLC) to produce predominantly DOC from tritiated pregnenolone, with no detectable aldosterone, in agreement with the in vivo results.