Granulomatous mastitis (GM) is a rare inflammatory breast disease of unknown etiology. Although it usually presents with sinus formation and abscesses, it may mimic the clinical characteristics of breast cancer. The aim of this study was to identify the clinical, radiologic, and pathologic characteristics of patients with GM and to show the results of surgical treatment in these patients. A chart review was performed for patients that were treated with a diagnosis of GM at the Breast Unit, Department of Surgery, Istanbul Medical Faculty, University of Istanbul, between September 1998 and January 2003. Eighteen patients were eligible for this study. The median age was 41.5 years (range 16-80 years). Seventeen patients were evaluated by both ultrasonography and mammography; whereas one young patient only had ultrasonography. Three patients were further examined with color Doppler ultrasonography and magnetic resonance imaging (MRI). Fourteen patients (78%) presented with a mass as the chief symptom, with a median size of 3.9 cm (range 1-8 cm), whereas four patients presented with fistula in their breasts. None of the radiologic techniques distinguished benign disease from cancer in any of the 14 patients that presented with a mass except one patient with normal mammography findings. Ultrasonography was only helpful to localize the abscess associated with a fistula tract in one patient. Therefore fine-needle aspiration biopsy (FNAB) was performed in six patients, followed by surgical excisional biopsy. The remaining eight patients with a clinical suspicion of malignancy underwent wide surgical excision with frozen section analysis under general anesthesia. All of the FNAB and frozen section evaluations revealed benign findings. All of the 18 patients underwent a wide excisional biopsy and had a definitive histopathologic diagnosis of GM. The median follow-up was 36 months (range 6-60 months). Only one patient had a recurrent disease, which was diagnosed at 12 months. GM is a rare breast disease that mimics cancer in terms of clinical findings. Preoperative radiologic diagnosis might be difficult. Complete surgical excision is the treatment of choice.