A case of unilateral adrenal adenoma with foci of oncocytes in a 64-year-old woman is reported. The tumour manifested clinically pre-Cushing's syndrome with autonomous steroidogenesis which was insufficient to cause typical features of Cushing's syndrome. Whether the pathogenesis of oncocytes and adenoma cells, which were thought to have some metabolic deficiency in the present tumour, are linked remains to be determined.