Decreased fertility is one of the characteristics of Turner syndrome. Ovarian function in women with Turner syndrome is believed to be impaired because of an abnormal and very rapid maturation of oocytes and follicles. About 30% of mosaic Turner patients have partial ovarian function during puberty, but only 2-5% of them will ever become pregnant. We describe a woman with a mosaic form of Turner syndrome (45,X [6]/146,XX [94] karyotype from blood lymphocytes), who had a spontaneous puberty and normal fertility. After her second pregnancy, she gave birth to a set of monozygotic female twins; Twin B presented with a mild Turner syndrome phenotype and Twin A with a normal female phenotype. Karyotypic analysis performed on amniotic fluid and fetal blood samples demonstrated a normal 46,XX chromosome constitution in Twin A and a 45,X/46,XX mosaicism (27%:73% for amniotic fluid and 6%:94% for fetal blood) in Twin B. Postnatal cytogenetic investigation of blood lymphocytes showed the 45,X [7]/46,XX [93] mosaicism in Twin B. Further investigations of blood lymphocytes in both girls at the age of 4 years showed Twin A with a 46,XX karyotype and Twin B with a 45,X [4]/46,XX [96] mosaicism. The phenotype of twin A had a normal appearance, but webbing of the neck, low posterior hair line, shield chest, and mild psychomotor retardation were evident in Twin B.