Context: Therapeutic management of primary aldosteronism requires accurate differentiation between aldosterone-producing adenoma (APA) and idiopathic hyperaldosteronism (IHA). However, little is known about the molecular features that delineate the difference between APA and IHA. Two different isoforms of 3β-hydroxysteroid dehydrogenase (HSD3B1 and HSD3B2) are thought to be expressed in the human adrenal gland, but the lack of isoform-specific antibody has so far hampered mapping of these isoforms in APA and IHA. Objectives: The aim of our study is to develop and characterize isoform-specific monoclonal antibodies against HSD3B1 and HSD3B2. Using these antibodies, we determined for the first time the immunolocalization of HSD3B1 and HSD3B2 in normal human adrenal cortex as well as in adrenal specimens from APA and IHA. Results: Immunohistochemical analysis with isoform-specific antibodies revealed zone-specific expression of HSD3B1 and HSD3B2 in the adrenal cortex. HSD3B1 immunoreactivities were essentially confined to the zona glomerulosa (ZG), in which aldosterone is produced. In contrast, HSD3B2 was not confined to the ZG but was found across the zona fasciculata, which is where cortisol is produced. Moreover, immunohistopathological analysis of primary aldosteronism revealed a previously uncharacterized difference between APA and IHA. Notably, hyperplasia of ZG seen for IHA was accompanied by a robust expression ofZGisoform HSD3B1. In contrast, tumor cells inAPAwere not immunopositive to HSD3B1. Rather, a strong and dominant expression of HSD3B2 characterized APA. Moreover, perhaps due to compensatory responses to excess aldosterone, APA had an adjacent ZG whose immunoreactivities to HSD3B1 and HSD3B2 were profoundly reduced. Conclusions: Isoform-specific monoclonal antibodies against HSD3B1 and HSD3B2 may be of great value for immunohistochemical differentiation betweenAPAand IHA.