A benign virilizing adrenal adenoma is rare among adrenal neoplasms in middle-aged women. A 39-yr-old Japanese woman who presented with hirsutism, obesity, diabetes mellitus and hypertension was admitted. Plasma concentrations of testosterone and DHEAS were high. While the basal level of plasma ACTH was suppressed, serum cortisol level was high and its circadian rhythm was absent. Serum cortisol level was not suppressed with the low- and high-dose overnight dexamethasone suppression test. Abdominal computed tomography showed a left adrenal tumor, and an adrenocortical scintigraphy revealed uptake of the tracer on the left side. Polycystic ovaries were also found and bone mineral density revealed osteoporosis. Histopathological features of resected adrenal tumor were consistent with those of adrenocortical adenoma. Immunoreactivity of all the steroidogenic enzymes was apparent in the tumor cells and particularly dehydroepiandrosterone sulfotransferase (DHEA-ST) immunoreactivity was markedly expressed. Cortical atrophy and reduced expression of DHEA-ST were detected in the cortex of the adjacent non-neoplastic adrenal gland. Plasma testosterone, DHEAS and cortisol levels returned to normal after surgery, concomitantly with the disappearance of polycystic ovaries. This is a very rare case of virilizing adrenocortical adenoma complicated with Cushing's syndrome (CS).