OBJECTIVE Ketoconazole treatment of Cushing's syndrome has been reported in single cases and a few small groups of 5-8 patients. We report our experience in 34 patients. METHODS Clinical study, with pretreatment and post-treatment evaluations. METHODS Out of 67 patients with Cushing's syndrome admitted during the last 6 years, 34 (28 females/six males; age range 14-67 years) received ketoconazole as a palliative treatment due to severe clinical conditions or management of the disease while awaiting results of definitive therapy. METHODS Urinary cortisol, plasma cortisol and ACTH, and routine chemistry were measured every week for 4 weeks, and then once a month. RESULTS Comparing the last values (mean +/- SEM) during treatment with baseline, urinary cortisol decreased from 1296 +/- 176 to 270 +/- 69 nmol/d (n = 34; P less than 0.001); plasma cortisol decreased from 672 +/- 31 to 549 +/- 35 nmol/l (n = 34; P less than 0.001). For patients with pituitary-dependent Cushing's syndrome, urinary cortisol decreased from 1073 +/- 126 to 200 +/- 21 nmol/d (n = 28; P less than 0.001) while plasma ACTH changed from 12.5 +/- 1.3 to 11.3 +/- 0.8 pmol/l (n = 26; not significant). Twelve patients were treated for more than 6 months, and those with pituitary-dependent disease all received pituitary radiation therapy, except the two who eventually escaped pharmacological control. One additional patient with adrenal carcinoma and one with ectopic ACTH syndrome showed lack of control of urinary cortisol levels. Ketoconazole was withdrawn within the first week in two patients for allergic reaction and acute liver toxicity. Other side-effects included: asymptomatic liver function abnormalities in three patients; gastrointestinal symptoms in four; worsening of gynaecomastia in one. Rapid clinical improvement was observed together with the normalization of urinary cortisol levels, with regression of symptoms such as diabetes mellitus, hypertension, hypokalaemia, and restoration of well being. CONCLUSIONS These data confirm that ketoconazole is valuable in the management of hypercortisolism, provided that patients are closely watched to exclude those who may develop liver toxicity and to prevent the occurrence of adrenal insufficiency.