Duplication of chromosome 15 (inv dup[15] chromosome) is the most common supernumerary marker chromosome in humans. Inv dup(15) chromosomes are commonly associated with mental retardation, epilepsy, behavioral problems and structural malformations. Ten patients (4 male, 6 female) were detected with inv dup(15) syndrome. At clinical follow-up three girls showed pubertal disorders: two with central precocious puberty and one with ovarian dysgenesis. As has already been found in other patients with chromosome 15p abnormalities, we believe that gynecological disorder is an important clinical finding also in patients with inv dup(15) syndrome. We report the first data of a systematic endocrinological study on inv dup(15) syndrome which suggest that endocrine investigation in these patients is both warranted and useful. Moreover, our observations confirm that a karyotype analysis in patients in whom precocious puberty is associated with mental retardation is mandatory.