PURPOSE: Men with Klinefelter syndrome have one or more extra X chromosomes and 
have endocrine abnormalities. Klinefelter syndrome has been consistently 
associated with breast cancer in men (MBC).
CASE REPORT: We report a 54-year old man was diagnosed as synchronous bilateral 
breast cancer with Klinefelter syndrome. On clinical examination there was mass 
in the lateral upper quadrant right breast. The overlying skin was slightly 
retracted. In the left breast, there was also a subareolar mass. Mammography, 
ultrasonography imaging showed bilateral suspicious breast masses with 
microcalcifications. There were no radiological findings of muscle invasion or 
axillary lymphadenopathy. We performed bilateral fine-needle aspiration biopsy 
(FNAB), and the aspiration smears were positive for carcinoma. The pathologic 
diagnosis of infiltrating ductal carcinoma in the biopsy specimen on the 
bilateral breast. The patient was successfully treated by bilateral radical 
modified mastectomy according to Madden's technique followed by external 
irradiation and adjuvant endocrine therapy.
CONCLUSION: Breast cancer commonly occurs in women, but now the incidence is 
also seen in men. Risk factors include age, family history, genes, liver 
diseases (cirrhosis), alcohol, diet, and obesity. Klinefelter syndrome, in which 
patients carry XXY chromosome, may be present in men with breast cancer for this 
reason they often develop gynecomastia.