Cancer cachexia is a common paraneoplastic syndrome in patients with advanced malignancies. However, the mechanisms of the development of cancer cachexia remain to be elucidated. Interleukin (IL)-6 is known to be involved in the development of cancer cachexia. The KPL-4 human breast cancer cell line, which was recently established in our laboratory, secretes IL-6 into the culture medium. Oral administration of doxifluridine (5'-DFUR, 60 mg/kg or 120 mg/kg) significantly inhibited the growth of KPL-4 tumors, reduced the tissue levels of IL-6, and alleviated body weight loss. Serum IL-6 levels were also lowered by 5'-DFUR in nude mice bearing KPL-4 tumors. Additionally, it is suggested that tumor necrosis factor (TNF)-alpha is involved in the cachexia induced by KPL-4 tumors. We suggest that 5'-DFUR suppresses cancer cachexia by lowering IL-6 levels in the tumor tissues and serum.