Plateau enrichment of 15N-ammonia following 24 hour continuous intravenous infusion of 15N-glycine was used to measure total body protein turnover and synthesis in normal volunteers and malnourished patients, with and without cancer. The mean postabsorptive total body protein synthesis rate in three normal controls was 2.5 g protein/kg/day. Protein synthesis and turnover decreased by a mean of 23% following one week of fasting, and returned to baseline levels following one week of refeeding. In three malnourished patients without known tumor, whole body protein synthesis and turnover was similar to controls; following seven to ten days of total parenteral nutrition (TPN) in the nontumor-bearing patients whole body protein synthesis and turnover decreased by 18%. Three of seven malnourished patients with known tumors had marked elevations in total body protein synthesis and turnover; TPN increased protein turnover in five of seven patients with known cancer. This study suggests that some malignant tumors can increase whole body protein synthesis and turnover in both the malnourished and fed state. This increase in protein turnover may represent a direct effect of the tumor, or reflect concomitant illness.