A 23-year-old woman with a 2-year history of discoid lupus (SLE) presented with a right lateral upper quadrant breast mass. Physical examination revealed a 5 cm irregular, hard lesion suggestive of a breast malignancy. Ultrasound-guided fine needle aspiration biopsy of the mass confirmed the diagnosis as lupus mastitis. Differential diagnosis of a breast mass in a patient with SLE must include the possibility of lupus mastitis. Surgical resection is usually not necessary, and medical treatment can be implemented successfully.