3 beta-hydroxysteroid dehydrogenase (HSD) deficiency was demonstrated in six males, aged between 18 and 24 years, who had gynaecomastia, hypogonadism or infertility. The predominant laboratory finding was a striking elevation of dehydroepiandrosterone sulphate (DHEAS) levels. The diagnosis of HSD deficiency was confirmed by finding a marked rise in dehydroepiandrosterone (DHEA) and 17-hydroxypregnenolone levels. In contrast to these findings in late-onset enzyme deficiency, in four males with the classical form of 21-hydroxylase deficiency the only sign was a reduction in adult height. The prevalence of late-onset HSD deficiency in men is not known and may be more relevant in patients with gynaecomastia or abnormal gonadal function than has hitherto been realized.