Unilateral adrenal hyperplasia (UAH) with primary aldosteronism is very rare, and shows similar endocrine features to aldosterone-producing adenoma and bilateral adrenal hyperplasia. In this study, the mRNA expression of steroidogenic enzymes in UAH was examined by in situ hybridization. We found subcapsular micronodules composed of spironolactone-body-containing cells, which showed intense expression for 3 -hydroxysteroid dehydrogenase, 11 -hydroxylase, 18-hydroxylase and 21-hydroxylase, but not 17 -hydroxylase, indicating aldosterone production. This expression pattern was the same as that in unilateral multiple adrenocortical micronodules, reported recently. Additionally, it was noted that a nodule with active aldosterone production was closely adjacent to one showing intense 17 -OH expression. In the adrenal cortices adhering to aldosterone-producing adenoma, the majority of hyperplastic zona glomerulosa and hyperplastic nodules demonstrated a decreased steroidogenic activity. However, minute nodules indicative of active aldosterone production were found at high frequency. These results suggest that the subcapsular micronodules observed might be the root of aldosterone-producing adenoma. Furthermore, we would like to emphasize the need for long-term follow-up after unilateral adrenalectomy or enucleation of the adenoma, because of the possibility that buds with autonomous aldosterone production may be still present in the contralateral or the remaining adrenal tissue.