The referral of a patient with features of Cushing's syndrome but with suppressed plasma cortisol and adrenocorticotrophic hormone concentrations prompted us to study the effect of medroxyprogesterone acetate (MPA) therapy on the adrenal axis. 11 women (aged 54-82 years) who were receiving 200-400 mg/day MPA were studied. Of these, four had subnormal plasma cortisol responses to a short synacthen test, and two more had borderline responses (30 min post-synacthen plasma cortisol results of 411 and 511 nmol/L). We conclude that suppression of the adrenal axis occurs relatively frequently in patients on MPA and that such patients should be checked for evidence of suppression before MPA therapy is withdrawn.