An autopsy case associated with both male breast cancer and hepatocellular carcinoma in which serum &agr;-fetoprotein (AFP) was extremely elevated is described. The source of AFP production was investigated. In lectin-affinity chromatography of the patient’s serum, concanavalin A (ConA) binding pattern was not consistent with that of hepatocellular carcinoma, and lens culinaris agglutinin A (LCA) affinity indicated a similar pattern to hepatocellular carcinoma. The immunohistochemistry staining for AFP revealed positive reactivity in breast cancer cells, but was negative in hepatocellular carcinoma cells. To the best of our knowledge, there have been no reports of breast cancer exhibiting immunohistochemically positive AFP in breast cancer cells. In the present case, ConA binding test was useful for identification of the source of AFP production. Furthermore, corresponding aggravation of breast cancer to the elevation of AFP suggested the possibility that AFP might enhance breast cancer growth.