Determination of the etiology of primary aldosteronism remains a diagnostic challenge. The most common types of primary aldosteronism are bilateral adrenal hyperplasia (BAH), aldosterone-producing adenomas (APA), and primary adrenal hyperplasia. Computed tomography (CT) and adrenal vein sampling (AVS) are the primary modalities used to differentiate these subtypes. The purpose of this study was to compare AVS and CT imaging of the adrenal glands in patients with hyperaldosteronism in whom CT imaging was normal or in whom focal unilateral or bilateral adrenal abnormalities were detected. The diagnosis of primary aldosteronism was made in 62 patients based on an elevated plasma aldosterone to PRA ratio and an elevated urinary aldosterone excretion rate. Thirty-eight patients had CT imaging and successful bilateral adrenal vein sampling and were included in the final analysis. AVS was considered the gold standard in determining the specific subtype of primary aldosteronism. There were 15 patients with APA, 21 patients with BAH, and 2 patients with primary adrenal hyperplasia. Plasma aldosterone was significantly higher in patients with APA (46.3 +/- 8.5 ng/dL; 1284 +/- 235 pmol/L) than in those with BAH (29.3 +/- 2.4 ng/dL; 813 +/- 11 pmol/L; P < 0.05). Plasma potassium was significantly lower in patients with APA (3.1 +/- 0.1 mmol/L) than in patients with BAH (3.5 +/- 0.1 mmol/L; P < 0.02). There was considerable overlap in the other biochemical indices (e.g. PRA and urinary aldosterone) in patients with the different subtypes. In patients with APA proven by AVS, eight had concordant findings with CT imaging, four had discordant findings, and three had normal CT imaging. In patients with BAH proven by AVS, four had concordant findings with CT imaging, eight had discordant findings, and nine had normal CT imaging. Compared with AVS, CT imaging was either inaccurate or provided no additional information in 68% of the patients with primary aldosteronism. We conclude that adrenal CT imaging is not a reliable method to differentiate primary aldosteronism. Adrenal vein sampling is essential to establish the correct diagnosis of primary aldosteronism.