Swollen lymph nodes (lymphadenopathy) are usually benign but can be a sign of breast cancer when the nodes are swollen in the armpit or near the chest wall. It is important to seek medical attention for lymphadenopathy that has been present for several weeks. Lymphadenopathy that is due to breast cancer will clear up once the cancer is treated.

Cancer spreads when cancer cells break away from the primary tumor. In breast cancer, these cells are most likely to reach the lymph nodes closest to the affected breast. Usually, these lymph nodes are under the arm, but there are also clusters of lymph nodes near the collarbone and breastbone.