We report three cases of idiopathic granulomatous mastitis that occurred in women of reproductive age (range, 25 to 36 years). These patients presented with breast masses of 2.5, 7, and 10 cm. One patient also had erythema nodosum. On biopsy specimens, the characteristic histopathologic features of granulomatous inflammation, centered on mammary lobules, were present in each case. In only one case was a specific diagnosis of granulomatous mastitis rendered based on the results of the biopsy; this patient received oral prednisone with prompt shrinkage of the breast mass to 20% of its original size. This residual mass was then resected. The remaining two patients were observed after biopsies were performed, with one experiencing a local recurrence after 5 weeks. The optimal therapy for idiopathic granulomatous mastitis has not been established, and we discuss treatment options, based on these three cases and those previously reported.