A calico-colored domestic shorthair cat was examined because of possible cryptorchidism. The cat had a fully formed penis, prepuce, and scrotum, but no descended testes, and exploratory laparotomy revealed a grossly normal female internal genital tract (ie, 2 ovaries, 2 uterine horns, and uterine body). Chromosomal analysis revealed a normal female (38,XX) karyotype. Four months later, the cat was examined because of polyuria, polydipsia, and inappropriate urination. Serum cortisol and aldosterone concentrations were low, and results of an ACTH stimulation test were suggestive of decreased adrenal gland function. Serum ACTH, testosterone, androstenedione, progesterone, 17-hydroxyprogesterone, 11-deoxycortisol, and deoxycorticosterone concentrations were high, and a diagnosis of congenital adrenal hyperplasia secondary to 11beta-hydroxylase deficiency was made. Treatment with prednisone diminished clinical signs but had a variable effect on corticosteroids hormone concentrations. To the author's knowledge, this is the first report of congenital adrenal hyperplasia in a cat.