Newborn monozygotic twins with phenotypic sexual discordance were diagnosed with 45,X/46,XY gonadal dysgenesis. Cytogenetic studies of peripheral blood (serial), skin, and gonad were performed. Serial comparative gonadotropin data and luteinizing hormone-releasing hormone stimulation tests were obtained. A phenotypically normal male twin was found to have a significant number of 45,X cells only in lymphocytes and was considered endocrinologically intact. The sexually ambiguous twin sister demonstrated a significant number of 45,X cell lines in all tissues and was considered endocrinologically agonadal. These data demonstrate the role of the 45,X karyotype on abnormal gonadal formation and function and make possible different etiologies for the 45,X cell line in both twins. While mosaicism is likely for both, chimerism is possible for the normal male.