In primary hyperaldosteronism, it is important to distinguish between unilateral and bilateral disease, as management strategies differ. In the period 1983-95, we identified 34 patients with primary hyperaldosteronism. Following further investigations, a diagnosis of aldosterone-secreting adenoma was made in 17 patients, and surgery was performed. Computed tomography clearly localized an apparent adenoma (discrete adenoma=1 cm diameter; normal contralateral gland) in only 10 of these patients (59%); two of these 'adenomas' were subsequently shown to be hyperplastic glands without adenomas. Histological examination showed adrenal adenomas in the remaining 15 patients. An 'adenoma' also appeared to be clearly localized in 3/17 patients later classified as having bilateral adrenal hyperplasia by adrenal vein sampling. CT scanning, therefore clearly localizes adenomas in only 50% of histologically proven cases, and can also produce misleading results. Adrenal vein sampling results altered our management approach in one third of cases. On the basis of our detailed results we would recommend surgery if there is clear evidence of unilateral aldosterone secretion along with CT findings which may not be strictly localizing but are in keeping with the dominant side on adrenal vein sampling. The decision to refer for surgery in primary hyperaldosteronism can be difficult, and we would caution against too heavy a reliance on CT results when recommending adrenalectomy, and suggest that adrenal vein sampling should remain a routine part of the investigation of patients with primary hyperaldosteronism.