Adrenocortical adenoma, nodular hyperplasia, or carcinoma was diagnosed in 50 ferrets. Thirty-five (70%) ferrets were female and 15 (30%) were male. The mean age at which clinical signs were first noticed was 3.4 years (range, 1 to 7 years). Clinical signs included large vulva (n = 31; 89% of females), alopecia (n = 43; 86%), pruritus (n = 20; 40%), and increased consumption of water and increased urine output (n = 4; 8%). A mass was palpated at the cranial pole of the kidney during physical examination of 17 (34%) ferrets. Ultrasonography, performed on 39 of 50 ferrets, revealed a unilateral adrenal gland mass in 19 (49%). Four ferrets were anemic, and 2 ferrets were thrombocytopenic. Baseline plasma concentrations of cortisol and corticosterone were within or below the reference range in all 17 ferrets tested, whereas baseline plasma estradiol concentrations were high in 4 of the 11 ferrets (36%) tested. AFter adrenocorticotropic hormone (ACTH) administration, only 1 ferret had a slightly exaggerated response on the basis of plasma cortisol concentrations, and all 17 had normal responses on the basis of plasma corticosterone concentrations. There was little or no increase in plasma estradiol concentrations after ACTH administration. Of the 50 ferrets, 39 were treated by adrenalectomy. Unilateral adrenalectomy was performed in 34 ferrets in which 1 adrenal gland was large, whereas subtotal bilateral adrenalectomy was performed in 5 ferrets with bilateral adrenal disease. Five ferrets died in the immediate postoperative period, and follow-up information was available for the remaining 34, 1 to 34 months after surgery.(ABSTRACT TRUNCATED AT 250 WORDS)