We report a case of idiopathic granulomatous mastitis in a 35-year-old Japanese woman, who came to our hospital complaining of a tender mass in her right breast. Because the results of 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 has tended to occur in young patients with a history of childbirth or oral contraceptive usage. Clinical or imaging diagnosis has often been 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.