We report a case of idiopathic granulomatous mastitis in a 30-yearold Iranian woman, who came to our hospital complaining of two tender masses in her right breast. Because the results of the initial aspiration cytology were considered highly suspicious for carcinoma, modified radical mastectomy was performed. However, the final histological diagnosis was granulomatous lobular mastitis with no evidence of malignancy. Idiopathic granulomatous mastitis is a rare inflammatory breast disease of unknown etiology. Since the clinical manifestations are similar to those of mammary carcinoma, this condition has been misdiagnosed as carcinoma and treated as such. A review of the literature revealed that idiopathic granulomatous mastitis tends to occur in young patients with a history of childbirth or oral contraceptive use. Clinical or imaging radiologic diagnosis is often difficult. Complete resection or corticosteroid therapy can be recommended as the optimal treatment. Since 38% of patients experience recurrence, long-term follow-up is indicated.