We report an extraordinary case of a collision tumor consisting of invasive ductal carcinoma with adjacent malignant lymphoma presenting as a single mass in the breast. A 79-year-old woman presented with a breast mass. A core biopsy performed at an outside hospital was interpreted as medullary carcinoma. On review of the breast core biopsy, a diagnosis of a synchronous malignant lymphoma and invasive ductal carcinoma was rendered. The patient underwent lumpectomy and axillary dissection. The excised specimen revealed a 2.1-cm, moderately differentiated invasive ductal carcinoma, partially surrounded by malignant lymphoma with areas where both tumors were intermixed. All 27 axillary lymph nodes were extensively involved by lymphoma, and 1 lymph node demonstrated metastatic carcinoma. The morphology and results of immunohistochemistry, flow cytometry, and cytogenetic analysis were consistent with extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue.