Mastitis is a disease common in the puerperal period and it only has a rare coincidence with breast cancer in nonpuerperal women. The characteristics of mastitis can masquerade as those of coincident breast cancer on the physical examination, mammography and breast ultrasonography. Fine needle as piration cytology might show cancer cells, but this not always possible. Therefore the detection of breast cancer coincident with mastitis is very difficult. Various researches have been conducted to see if mastitis is a risk factor for breast cancer. At present, there is no evidence to support a relationship between breast cancer and mastitis. The coincidence of breast cancer, and especially noninflammatory early breast cancer and mastitis is very rare, but the effect on the patient could be great. Attention should be paid to the differential diagnosis or for a coincidental finding of breast cancer.