A 36-year-old woman presented to our hospital with a rapidly growing lump in her left breast. Fine-needle aspiration (FNA) cytology 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. This report describes the clinical course of this patient in whom granulomatous mastitis was successfully treated with corticosteroid therapy. Special reference is made to the usefulness of FNA cytology in the diagnosis and follow-up of this disease.