To determine endocrine activity of adrenal tumours incidentally discovered on C.T., we examined 20 consecutive patients. They underwent thorough hormonal assessment and scintigraphic scanning with radioactive cholesterol under dexamethasone suppression (19 patients). Biochemical findings compatible with cortisol hypersecretion were detected in 5 patients. One patient had reduced reserves of cortisol secretion and one had hyporeninemic hypoaldosteronism. The scintigraphy showed no uptake in 10, unilateral uptake in 4, and bilateral uptake in 5 patients. In 3 patients the finding was unilateral on CT. but bilateral on scintigraphy. Signs of autonomous cortisol production were more common among patients who had uptake on scintigraphy. At the operation of 8 patients only benign lesions were found. During the follow-up (9 to 49 months) of the 12 unoperated patients, the tumour disappeared in one and remained unchanged in the others. No changes occurred in the biochemical findings except in one patient whose cortisol response to 1 mg of dexamethasone became abnormal. Since slight hypercortisolism or a bilateral disease often exists behind an incidentally discovered adrenal tumour, we emphasize the importance of careful assessment of cortisol metabolism and a scintigraphic scanning under dexamethasone suppression in the examination of these patients.