BACKGROUND Computed tomography (CT), magnetic resonance imaging (MRI), and adrenal vein sampling (AVS) are used to distinguish unilateral from bilateral increased aldosterone secretion as a cause of primary aldosteronism. This distinction is crucial because unilateral primary aldosteronism can be treated surgically, whereas bilateral primary aldosteronism should be treated medically. PURPOSE To determine the proportion of patients with primary aldosteronism whose CT or MRI results with regard to unilateral or bilateral adrenal abnormality agreed or did not agree with those of AVS. DATA SOURCES PubMed, MEDLINE, EMBASE, and Cochrane Library, 1977 to April 2009. STUDY SELECTION Studies describing adults with primary aldosteronism who underwent CT/MRI and AVS were included. Of 472 initially identified studies, 38 met the selection criteria; extractable data were available for 950 patients. DATA EXTRACTION The CT/MRI result was considered accurate when AVS showed unilaterally increased aldosterone secretion on the same side as the abnormality seen on CT/MRI or when AVS showed symmetric aldosterone secretion and CT/MRI revealed bilateral or no unilateral abnormality. DATA SYNTHESIS In 37.8% of patients (359 of 950), CT/MRI results did not agree with AVS results. If only CT/MRI results had been used to determine lateralization of an adrenal abnormality, inappropriate adrenalectomy would have occurred in 14.6% of patients (where AVS showed a bilateral problem), inappropriate exclusion from adrenalectomy would have occurred in 19.1% (where AVS showed unilateral secretion), and adrenalectomy on the wrong side would have occurred in 3.9% (where AVS showed aldosterone secretion on the opposite side). LIMITATION The lack of follow-up data in the included articles made it impossible to confirm that adrenalectomies were performed appropriately. CONCLUSION When AVS is used as the criterion standard test for diagnosing laterality of aldosterone secretion in patients with primary aldosteronism, CT/MRI misdiagnosed the cause of primary aldosteronism in 37.8% of patients. Relying only on CT/MRI may lead to inappropriate treatment of patients with primary aldosteronism.