Could it be cancer?

A lump in the breast should always cause some concern, though breast cancer in men is very unusual--less than 1%, and this is generally in older men. Gynecomastia often occurs on both sides, so if a breast mass occurs on only one side, it may be more suspicious for cancer. It would be extremely unusual to see male breast cancer on both sides at the same time. Sometimes a patient's history may give a clue as to whether or not the lump could be cancer. In other cases, a mammogram or ultrasound may be required. Sometimes a biopsy of the tissue must be done to be sure it isn't cancer.