We report a case of synchronous bilateral breast cancer in a patient with ambiguous external genitalia attributed to a 45,X/46,XY mosaicism. To our knowledge, this represents the first such case ever to be reported. Mammography, ultrasonography, computed tomography, and magnetic resonance imaging all showed bilateral suspicious breast masses with microcalcifications. There were no radiological findings of muscle invasion or axillary lymphadenopathy. The patient was successfully treated by bilateral radical modified mastectomy followed by external irradiation and adjuvant endocrine therapy. Histological examination revealed a bilateral ductal carcinoma in situ, with a cribriform and papillary pattern and microfoci of infiltrating ductal carcinoma. The hormonal profile revealed high levels of follicle-stimulating hormone and luteinizing hormone, and low levels of testosterone. Testicular sonography revealed small hypoechoic testicles with bilateral microlithiasis. This case shows that 45,X/46,XY men may have an increased risk of breast cancer and must be followed up carefully.