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Objectives: To prospectively investigate whether AVS is useful to identify the aetiology of excess aldosterone secretion in pts with the biochemical features of PA and equivocal CT and MRI findings. Design and Methods: Between 1992 and 1999, 104 referred hypertensive pts (46 f and 58 m; age: 49.3 ± 12 yrs) to our department were diagnosed as PA, based on supine (sAldo) and captopril-suppressed plasma aldosterone (cAldo), supine plasma renin activity (sPRA), serum K+, and a logistic multivariate discriminant analysis score. AVS for measurement of plasma aldosterone (Aldo) and cortisol (C) levels was performed in all since CT scan (n = 104) and MRI (n = 69) did not provide unequivocal identification of an aldosterone-producing adenoma (APA). It was judged to be selective and thus used for diagnostic purposes when the ratio between C levels in each adrenal vein and inferior vena cava plasma was >1.2. Results: Forty pts (38%) were found to have APA; the average tumor size, as assessed by CT, MRI and pathology, resulted to be 1.25 ± 0.52 cm. AVS resulted to be bilaterally selective in 78.1% of all cases, being selective on the right and left side in 94% and 86%, respectively. AVS provided a definite aetiologic diagnosis of PA in 88% of cases where it was selective. Adrenal vein rupture leading to loss of unilateral adrenal function, as assessed by scintigraphy, occurred in one pt (1%) with idiopathic hyperaldosteronism. Conclusions: AVS was feasible and safe in the vast majority of PA pts with equivocal or negative CT and MRI scan. When selective, it provided an accurate lateralization of the aetiology of PA.