Recently, a British woman, Kincaid, uploaded a photo of her breasts on social media, which triggered a new round of “breast” war. In addition, it has been rumored on the internet that women with bigger breasts have a much higher chance of breast cancer than those with flat breasts. Is there really a relationship between breast cancer incidence and breast size? A study by the Harvard School of Public Health, which surveyed 89,268 women between the ages of 29 and 47, concluded that “among premenopausal, slimmer women with a BMI less than 25 kg/m2, the incidence of breast cancer was higher in women with cup size D and above than in women with cup size A and below, but the association was higher in women with a BMI greater than 25. BMI greater than 25 in obese women was not significant.” The study suggests that the “greater risk of breast cancer with larger breasts” does hold true, provided certain conditions are met. In a study published on the Washington Post website, “5 Outrageous Claims About Breast Cancer,” the study was refuted. There is still a lot of controversy in the scientific community about breast size, including another study that concluded the opposite: that women with smaller breasts may have a higher risk of breast cancer due to muscle tissue density. Who to believe? There is simply no way to find out. The risk of breast cancer should not be focused on whether or not the breasts are large or small. The growth and development of the breast is mainly influenced by a variety of hormones in the reproductive endocrine axis, such as gonadotropins, lactogen, estrogen and progesterone. Thus, if endocrine disorders occur, the normal development of breasts will be affected, which can lead to cancer risk. In addition, the structure and number of ducts and glands in the breast are the same regardless of the size of the breast. The theory that women with small breasts have fewer glands and therefore have less chance of breast cancer is not valid. Therefore, experts suggest that regardless of breast size, women must maintain a good lifestyle, stay relaxed, and not use exogenous estrogen indiscriminately to prevent breast cancer. Among the lifestyle changes that have been proven to be effective in reducing the risk of breast cancer, including getting pregnant at the right time, adhering to breastfeeding, and 1-3 hours of aerobic exercise per week can reduce the risk of breast cancer by 18%; in addition, lifestyle changes such as controlling weight, reducing fat intake, taking enough vitamins, and smoking and passive smoking can all reduce the risk of breast cancer.