A hypernephroma removed from a male patient who had lost 30 kg in weight in the 2 months preceding surgery was established in immunosuppressed CBA/Lac mice as a nonmetastasizing transplantable xenograft. The xenografted tumors, although comprising less than 5% of the total body weight of the mice, produced considerable weight loss (greater than 25%). A slight reduction in food intake of tumor-bearing mice was noted, but some animals bearing mouse or human tumors not inducing cachexia had equally low food intake without accompanying weight losses. No obvious defects in gastrointestinal histology or absorption were observed. The precise mechanism(s) producing the severe cachexia remains to be established.