In a patient with primary aldosteronism, in which the postural endocrine tests suggested the presence of an aldosteronoma rather than hyperplasia, bilateral adrenal tumours were found by computer tomography. Adrenal scintigraphy using 6-131I-iodomethyl-19-norcholesterol (NP59) during dexamethasone suppression showed early unilateral adrenal visualization on the left side. After removal of the left adrenal gland, which contained a 2 x 2 x 2 cm adenoma, the blood pressure and aldosterone levels returned to normal. A CT-scan, performed 1 year after the pre-operative CT-scan, showed no change in size of the right adrenal tumour, consistent with a non-functioning adenoma. In this patient, the NP59 scan adequately distinguished a non-functioning from an aldosterone-producing adrenal tumour.