A 36-yr-old woman was referred to the endocrine clinic for investigation of oligomenorrhea, hirsutism, and acne. She was plethoric and overweight with central fat distribution. Plasma cortisol was normal, but her adrenal glands were enlarged (CT scan). Urinary tetrahydrocortisone excretion rate was consistently high, raising the possibility of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) deficiency. In addition, 5beta- reduction of cortisol and cortisone was markedly enhanced. The levels of all cortisol metabolites were suppressed normally with dexamethasone, but conversion of oral cortisone acetate to plasma cortisol was delayed and subnormal compared with that of healthy volunteers. This was accompanied by a larger than normal increase in plasma cortisone concentration. Thus, the defect appears to be in 11beta-HSD1 activity and not in 5beta-reductase activity. Three close relatives of the subject showed no comparable abnormalities, and analysis of the coding region and exon/intron boundaries of the 11beta-HSD1 gene of the case revealed no differences from the consensus sequence. The defect may lie outside the coding region. Alternatively, some other inherited or acquired defect may lead to inhibition of this enzyme system.