A 36 year old woman, mother of a two year old child developed, in the course of one night, a tender mass in the upper medial quadrant of the left mamma. Treatment with antibiotics had no effect, and after a week the patient was admitted to hospital for drainage of the abscess and further examination. She had then developed reactive arthritis. Histological examination of a specimen from the mamma revealed lobular granulomatous mastitis. This connection has not been described before in the literature. Further examination showed no signs of infectious disease or sarcoidosis. Surgical drainage had only a minor effect on the breast-mass. The patient was treated with prednisone for six months, and after one year of observation the mass has disappeared, but the arthralgias persists.