Background: Granulomatous mastitis, a rare, benign breast condition characterized by granulomas and abscess formation, is mistaken frequently for inflammatory breast carcinoma. Although it has been reported in reproductive-age women, it is unusual in pregnancy. Case: A 25-year-old black gravida 4 para 2 developed a tender, indurated 20 x 15 x 15 cm multilobulated breast mass at 17 weeks' gestation. After not responding to antibiotics and incisional drainage, the biopsy-proved granulomatous mastitis improved with corticosteroid treatment. A postpartum recurrence also responded to steroid therapy. Conclusion: Granulomatous mastitis is diagnosed clinically and histologically. Early recognition and initiation of steroid treatment might prevent repetitive, deforming breast biopsies. (Obstet Gynecol 2000;96:813-5. (C) 2000 by The American College of Obstetricians and Gynecologists.).