In order to define the role of scintigraphy in determining the aetiology of primary aldosteronism, 41 patients were examined by computed tomography (CT) scan and adrenal scintigraphy using iodine-131 6beta-methyl-iodo-19-norcholesterol with the dexamethasone suppression test. Hormonal and scintigraphic examinations were conducted while avoiding interference by medical treatment. The aetiological diagnosis was established by taking account of the clinical context, the endocrine profile, and CT scan and scintigraphic data, as well as possible hormone assays after catheterization of the adrenal veins (12 cases) and postoperative pathology data (14 cases). The aetiological diagnoses established were Conn's adenoma (insensitive to angiotensin II) in 12 cases, idiopathic hyperplasia in 11 and macronodular hyperplasia (with functional autonomy of the nodules) in 18. Unilateral and bilateral lesions were correctly distinguished by scintigraphy in 92% of cases as compared with only 58% using CT scan alone; this was because the CT scan appearance was normal in 3/12 cases of adenoma and because a single nodule was visible in 2/11 cases of idiopathic hyperplasia and in 12/18 cases of macronodular hyperplasia. It is concluded that scintigraphy using noriodocholesterol with the dexamethasone suppression test should be performed systematically in conjunction with hormonal tests and adrenal CT scan in all cases of primary aldosteronism, as part of a strategy aimed not only at detecting adenoma but also at determining whether the hyperfunctional lesions are bilateral.