Idiopathic granulomatous mastitis (IGM) is a rare chronic inflammatory lesion of the breast clinically simulating carcinoma. The purpose of this report is to review the clinicopathological features of this clinical entity in a series of 18 cases diagnosed in our institute. The notes of patients and the slides of the biopsy specimens of 18 cases with IGM were reviewed. Special stains for organisms and immunohistochemistry for T and B markers using the primary antibody was done. The diagnosis of IGM was made according to the criteria of Kessler and Wolloch. All but two patients were of reproductive age and all were parous. The main clinical finding was a unilateral, firm, discrete mass. All patients underwent excisional biopsy and recurrence was seen in three patients. In two of these patients who had recurrences, the prolactin level was high and reexcision combined with antiprolactinemic therapy was performed. The third patient was treated by reexcision and oral prednisone. There was granulomatous inflammation, centered mainly on breast lobules in all patients. T cell domination was observed. Exclusion of the other causes of a granulomatous lesion is necessary to make the diagnosis of IGM. The treatment of choice is unclear. Because of the clinical concern of malignancy, fine-needle aspiration cytology or core biopsy can be useful in some cases. To differentiate it from other granulomatous lesions, incisional biopsy or surgical excision of the lesion is necessary. In patients with delayed wound healing or recurrence after excisional biopsy, or those patients who have had an incisional biopsy only, if prolactin level is normal, reexcision and oral prednisone usage may be curative. In patients with a high prolactin level who have recurrence, medical treatment to control prolactin level may be useful in the management of these women.