There is a high risk of neoplasm in dysgenetic gonads. Classically, in Turner syndrome, only patients with 45, X/46, XY mosaicism karyotype or with a fragment of Y chromosome (45, X+mar) are at risk of developing gonadal tumor. A case of a dysgerminoma arisen on the dysgenetic gonad of a 45, X Turner patient in the absence of Y chromosome material at cytogenetic and molecular biology analysis is reported. Five cases of gonadal tumor with a pure 45, X chromosome constitution have been previously reported in the literature. In these cases only cytogenetic analysis was performed. This is the first case of an ovarian tumor in a 45, X Turner syndrome in which the presence of Y material can be ruled out by an extensive molecular analysis of the blood and the tumor.