The patient in this study was a 39-year-old Chinese woman who presented with a painful right breast lump on April 5, 2008. She had a history of schizophrenia for over 20 years and had been receiving risperidone (2 mg bid) for more than 3 years. She had developed a lump in her right breast a month before her visit and had noted a progressive worsening tenderness with erythematous changes around the lesion. She had no systemic symptoms nor did she have any known previous fungus or tuberculosis exposure. The patient had a gestational history of G0P0 and had amenorrhea for approximately a year. She had no history of any other breast disease or receiving any breast surgery, and her medical history was otherwise unremarkable. She had never consumed tobacco, alcohol, oral contraceptive pills nor did she have any family history of breast cancer. On physical examination, a 9-cm x 6-cm breast lump with localized redness over the right breast lateral aspect with an ill-defined margin was noted with no palpable lymphadenopathy at the axilla. There was no splenomegaly or hepatomegaly. She did not have fever, joint pain, airway or urinary tract bleeding. No other skin lesion was found. The patient's blood count was normal. Breast ultrasound images revealed a large ill-defined area with heterogeneous echoes in the right upper and lower outer quadrants, associated with increased vascularity. Micro-calcification and tissue edema were noted. No enlarged lymph node was noted in the right axilla, and there was no dilatation of the lactiferous duct. CXR didn't show any lung lesions. Mammography showed asymmetry with increasing radiodensity at the outer upper quadrant of the right breast with nipple retraction. Mastitis with edema and micro-abscess in the right upper and lower quadrants was suspected. However, inflammatory carcinoma could not be ruled out. All other laboratory and other radiological studies including C-reactive protein (0.8 mg/L) were normal except that the prolactin level was 84.5 ng/ml (normal, < 20 ng/ml); FSH, 4.6 mIU/ml; and LH, 6.0 mIU/ml. Although malignancy had not been excluded, the patient received a presumptive diagnosis of infectious mastitis and was treated with a 14-day course of keflex (first-generation cephalosporin), but there was no improvement. Therefore, core breast biopsy was performed, which revealed an adipose tissue with acute and chronic inflammation. Aspiration culture showed no evidence of bacterial or mycobacterial growth. In order to rule out previously missed diagnosis, another incision biopsy was performed 1 week later, and the pathology showed chronic inflammation with focal fibrotic changes in the fat tissues. Due to persistent drainage with erythematous swelling and in consideration of malignancy, the patient then was scheduled for simple mastectomy. Macroscopic appearance demonstrated a huge mass with an ill-defined margin of inflammatory tissue at the center with peripheral fatty necrosis and hematoma (Figure 1). Microscopic examination revealed chronic inflammation and macrophage, giant histiocyte, and epithelioid-like cellular infiltration, with cytologic features suggestive of a granulomatous process. The noncaseous granulomatous lesions were centered at the breast lobules. The lesions were characteristic with an empty space in the center and surrounded by microgranulomas and microabscesses. Some granulomatous lesions composed of confluent epithelioid cells and huge abscess were also seen (Figure 2A, B). Based on the histological features, the differential diagnosis included autoimmune response, undetected organisms, systemic granulomatous disease with breast involvement, granulomatous reaction in a carcinoma and foreign body reaction. There was no evidence of carcinoma or specific organism can be found. All cultures, Ziehl-Neelsen staining, PAS staining and GMS staining yielded negative results. Considering the patient's clinical history and all the laboratory findings, the diagnosis was interpreted as IGM. In suspect of the dopaminergic effect of risperidone to cause hyperprolactinemia and IGM, the prescription was shifted to prolactin-sparing second line agent (clozapine). The residual breast lesions resolved completely 3 months later and no recurrence was noted since then.