The mosaic karyotype of 45,X/46,XY has a wide phenotypic spectrum and there are substantial differences between prenatally and postnatally diagnosed cases. The phenotype varies between normal male to classical Turner syndrome. There is a high risk of gonadal tumor development in the dysgenetic gonads of patients with sex chromosome mosaicism. We report a case of a 24-year-old patient with a pelvic mass and amenorrhea referred to our laboratory for karyotyping. Peripheral blood chromosome analysis showed a mosaic karyotype of 45,X/46,XY. The tumor originated from the left ovary and the right ovary was found to be a streak gonad. The uterus was intact. Pathologic examination of the tumor revealed mucinous cystadenoma. Physical examination of the patient showed signs of Turner syndrome, as short stature (145 cm), short neck and asymmetric shoulders. Her mental state was normal. Y chromosome microdeletion screening involving SRY and ZFY genes was performed and no deletion was found. The patient was informed about the condition during the genetic counseling session.