Objective: Idiopathic granulomatous mastitis (IGM) is a disfiguring inflammatory breast disease without effective treatment. We report the largest IGM cohort treated with methotrexate (MTX) monotherapy.Methods: Chart review was performed on patients evaluated by the Stanford Immunology and Rheumatology Clinic, with histopathologically established IGM treated with MTX, and at least 1 followup appointment. Results: Nineteen female patients with a mean age of 33.5 years were identified. Most failed treatment with antibiotics, prednisone, and surgical intervention. By 15 months of treatment with MTX, 94% had disease improvement and 75% achieved disease remission.Conclusion: MTX monotherapy is an effective treatment for IGM.