Late-onset congenital adrenal hyperplasia due to 3 beta-hydroxysteroid dehydrogenase deficiency has been reported with increasing frequency, but only a few adult women have been found to have this disorder in Japan. We report a 26-year-old Japanese hirsute woman with partial 3 beta-hydroxysteroid dehydrogenase deficiency. The diagnosis was based on significantly increased ratios of 17-hydroxypregnenolone to 17-hydroxyprogesterone and of dehydroepiandrosterone to androstenedione after administration of ACTH. Hirsutism improved with the administration of dexamethasone (0.5 mg) every evening. Since routine assay of delta 5-steroid metabolites has become available, the incidence of this disorder will increase. Diagnostic effort should be attempted since the disorder is treatable with low-dose dexamethasone.