A 46-year-old woman presented to our hospital with a rapidly growing lump in her right breast. Fine-needle aspiration (FNA) cytology and core needle biopsy of the mass revealed many epithelioid cells admixed with multinucleated Langhans-type giant cells, neutrophils, lymphocytes, and stromal cells, leading to a diagnosis of granulomatous mastitis. Mammography and ultrasonography provided little information for differentiating between granulomatous mastitis and carcinoma. This patient was successfully treated with low dose and short period of corticosteroid therapy.