Sometimes the normal fat cells in the breast become round, firm lumps made up of damaged fatty tissue. This is called fat necrosis of the breast. The lumps may or may not be painful. This problem is most often seen in obese women who have very large breasts or after an injury to a breast.

Fat necrosis can be caused by an injury or blow to the breast. Sometimes the skin around this lump looks red or bruised. Fat necrosis is sometimes seen following a motor vehicle accident in which the seat belt has forcefully squeezed the breast.