PURPOSE: Idiopathic granulomatous mastitis (GM) is an uncommon breast disease that mimics carcinoma in terms of the clinical and radiological findings. Its unknown etiology makes the management difficult. Although surgical excision with or without steroid therapy has been the mainstay of the treatment, the efficacy of this treatment has been inconsistent. This study was performed to evaluate the diagnostic methods and the treatment outcomes. METHODS: This retrospective study included 31 patients with the diagnosis of GM and 4 patients had bilateral disease. They were found from the database of the pathology laboratory at Cheil General Hospital between January 1999 and December 2004. We reviewed the clinical, radiological, and pathological information, and we finally evaluated the treatment outcomes according to the surgical and medical treatments. RESULTS: The mean age at diagnosis was 34.2 years (range: 27~57). All the patients were parous except one, and 82% of patients were within 5 years from their last delivery. Four patients had bilateral involvement. Only three patients used oral contraceptives and 54% of patients had a history of breast feeding. Most patients (91%) presented with mass and the remainder (9%) presented with fistula. The confirmative diagnosis was made by FNAB (fine needle aspiration biopsy), debridement, CNB (core needle biopsy) or excision. Surgical excision or debridement was successful in 13 cases (39.4%) of 33 cases. Anti-tuberculosis medications were successful in 85% (11/13). Steroid treatment was successful in only two patients (29%). CONCLUSION: Surgical excision provided not only the definite diagnosis, but also treatment of GM for the cases in which a clear surgical margin was obtained. Anti-tuberculosis medication can be considered if fistula or abscess is intractable after excision, even with failure to identify tuberculosis.