This retrospective study was aimed 1) to compare the difference of the findings between adrenal CT scan and adrenal venous sampling (AVS) in 35 cases with definite primary aldosteronism (PA) for assessment of the diagnostic efficacy of PA subgroup (unilateral and bilateral adrenal hypersecretion: UAH and BAH), and 2) to determine the clinical and biochemical parameters as potential predictors for PA subgroup. There were significant discordant results based on AVS and CT scan in subgrouping PA; 9 of 17 BAH patients (53%) had unilateral lesion on CT scan, while 4 of 18 UAH patients (22%) had no apparent or bilateral lesions on CT scan. Among three diagnostic criteria, absolute values of plasma aldosterone concentration (PAC) in both adrenal veins, lateralized and contralateral ratios of aldosterone/cortisol after ACTH stimulation during AVS to determine the laterality, none of them showed 100% diagnostic accuracy if applied alone. Among several clinical and biochemical parameters, hypokalemia (<3.4 mEq/l), younger age (<52 y) and poor response of PAC (<1.45) after furosemide-upright posture, proved to be significant predictors for UAH, with higher specificities (100%, 88%, 94%, respectively). Therefore, despite AVS as a gold standard method to determine the laterality of aldosterone hypersecretion in PA, our study shows that no single criterion could provide definite diagnostic value for its laterality by AVS. It is also suggested that most PA patients, if not all, with a distinct unilateral adrenal lesion on CT accompanied by hypokalemia, younger age and poor aldosterone response to renin stimulation, could undergo adrenalectomy without prior AVS.