Background:Chromosomal mosaicism is characterized by the presence of two or more distinct cell lines in an individual. Mosaicism in sex chromosome is a major component of Disorders of Sex Development (DSD) results in a large clinical spectrum of genital ambiguity. Case Presentation:We report long term follow-up of a 15-year-old male who was evaluated for ambiguous genitalia with a karyotype of 46,XY (85%) / 46,XX (15%). He presented with abnormal urethral opening (hypospadias) and left sided undescended testis since birth. Work-up was done for cytogenetic analysis, hormonal assays, imaging, exploratory laparotomy, and hypospadias repair. For more than 15 years he was reared as a boy, with no further complaints, until he reached puberty. He then developed gynecomastia and monthly painful hematuria. MRI evaluation revealed a left adnexal cystic mass and anteflexed uterus with loculated fluid collection posterior to urinary bladder suggesting hematometra. We discuss the genetics, diagnostics, as well as genetic counseling of this patient. Conclusion: This case is reported in view of the interesting clinical presentation of this rare mosaicism. A strong emphasis on a multidisciplinary approach and close follow-up is important to ensure both physical and psychological well-being of DSD patients.