It is impossible to generalize about the seriousness of breast cancer less than 1 cm. It is usually not serious if the treatment is timely and the tumor has a good pathological type, while it is more serious if the treatment is untimely and the tumor has a poor pathological type. Breast cancer is a malignant tumor disease that originates in the epithelial cells of the breast. Determining whether breast cancer less than 1 cm is serious or not needs to be determined on a case-by-case basis and cannot be generalized. If patients with breast cancer less than 1 cm are treated timely and the tumor pathology type belongs to adenotubular carcinoma, mucinous adenocarcinoma, etc., it is generally not serious and has a better prognosis. However, if the treatment of breast cancer patients less than 1 cm is not timely, resulting in further progression of the disease and belonging to poorer tumor pathology categories such as pyogenic breast cancer and imprinted ring cell carcinoma, they are relatively more serious and have a poor prognosis. Therefore, whether breast cancer is serious or not is mainly closely related to the development stage of the disease, and the earlier it is detected, the better its prognosis. To sum up, patients with breast cancer less than 1cm need to go to the hospital in time, cooperate with doctors for active treatment, strengthen the management of daily life, as well as need regular review and follow-up to avoid delaying the disease.