Using a single dose, [15N]glycine turnover technique, whole body rates of proteins synthesis and breakdown were assessed in six healthy children and in eight children with newly diagnosed leukemia (DeWys, W. D. Cancer Res., 42: 721s-726s, 1982) or lymphoma (Baracos, V., Rodemann, H. P., Dinarello, C. A., and Goldberg, A. L. N. Engl. J. Med., 308: 553-558, 1983). Based on excretion of 15N as urinary ammonia, synthesis (g protein per kg body weight per day) was significantly (p less than 0.025) higher in the cancer patients [5.4 +/- 1.5 (S.D.)] compared to the controls (3.6 +/- 0.9); breakdown was also higher (p less than 0.02) in the patients (5.5 +/- 1.8) compared to the controls (3.1 +/- 1.1). When only the seven patients with leukemia were considered, there also were significant increases in synthesis (5.4 +/- 1.6, p less than 0.05) and breakdown (5.4 +/- 1.9, p less than 0.025) compared to controls. Increases in both synthesis and breakdown were also observed in the patients when the protein turnover data were expressed as a function of the rate of creatinine excretion or the calculated lean body mass. We conclude that whole body protein turnover is increased in sick children at the time of diagnosis with some forms of newly diagnosed cancer.