Invasive lobular breast carcinoma is an exceedingly rare neoplasm in men, representing a very small percentage of all male breast cancers. However, it should remain on the differential diagnosis in a patient with metastatic carcinoma of unknown origin. We report a case of a 58-year-old man who presented with increasing abdominal girth and weight loss. A peritoneal biopsy revealed a malignant epithelial neoplastic process focally arranged in a single-file pattern. There was immunohistochemical reactivity for cytokeratin AE1/AE3. E-cadherin, cytokeratin 5/6, prostatic-specific antigen, and leukocyte common antigen immunostains were negative. Subsequent gastric and right iliac bone biopsies revealed a poorly differentiated carcinoma similar in histology and immunophenotype to the previous peritoneal biopsy. Upon a more thorough physical examination, bilateral breast masses and nipple retraction were noted. Biopsies of the masses revealed estrogen receptor-positive, E-cadherin-negative invasive lobular carcinomas with similar histology to all the previous biopsies. This case is unusual in that it represents the first reported case of lobular carcinoma of the male breast presenting as carcinomatosis. The fact that the patient was male prolonged the identification of the primary site. Although a rare neoplasm in a male, breast cancer should be included in the differential diagnosis of a metastatic carcinoma of unknown origin.