A 23-year-old woman was admitted with complaints of swelling and pain in the left breast, fever, polyarthralgia and erythema nodosum. A fine-needle biopsy of the mass in the left breast revealed non-caseous granulomatous lobulitis. A diagnosis of granulomatous mastitis was thus made. The administration of prednisolone 40 mg/day resulted in the resolution of the patient's symptoms, and the breast mass thereafter decreased in size. The mass relapsed during the subsequent prednisolone taper. Additional therapy with methotrexate resulted in complete remission. Granulomatous mastitis should therefore be included in the differential diagnosis of polyarthralgia.