A 16 year old girl was evaluated for short stature, primary amenorrhea, and lack of development of secondary sex characterisics. She did not have the classical phenotypic signs of Turner's syndrome. However, she was short, had infantile genitalia and the adnexa were not palpable. There was a minor bone malformation and no signs of visceral abnormalities. The hormonal studies showed hypergonadotropic hypogonadism and borderline hypothyroidism. The rest of the endocrine functions was normal. The Barr bodies on the buccal smear were decreased and in part abnormal, large or bipartite. The karyotype showed mosaicism of about equal proportions of 45 X, 0/46, X, dic (X). The large dicentric chromosome was due to the end to end fusion of two X chromosomes by their short arms. Sequential binding studies were performed and failed to document any loss of genetic material of the dicentric X. It is speculated that the fusion of X chromosomes during an early mitotic division was responsible for the 45 X, U cell line, and that the short stature and gonadal dysgenesis in this patient was due to the presence of the 45 X, 0 line.