Incidental metastatic breast carcinoma was discovered by histologic examination in a total knee arthroplasty (TKA) patient. A 70-year-old woman was evaluated for increasing debilitating pain in the right knee. A radiograph demonstrated only degenerative changes. Degenerative changes were noted in the gross and microscopic examination of the knee specimen. The unsuspected focus of poorly differentiated adenocarcinoma was suggestive of mammary carcinoma. A mass in the right breast then was subsequently noted on reexamination. Mastectomy showed infiltrating adenocarcinoma with metastases to axillary lymph nodes, and diffuse involvement of the skeleton was demonstrated with a bone scan. This case emphasizes the importance of a meticulous history and a complete medical examination before surgery, detailed examinations of the excised knee and joint fragments by a pathologist after a TKA operation. Malignancy can be missed with only gross specimen evaluations and radiographs.