We compared the kinetics of urea production and leucine oxidation in severely malnourished Malawian children. We tested the hypotheses that the rate of urea production was directly proportional to the rate of leucine oxidation and that the relationship between the two is altered by acute infection. Thirty-six marasmic children, aged 12 to 60 months, were enrolled; 26 had acute infection and 10 did not. The rates of urea and CO(2) production were estimated using primed, constant, intravenous stable isotope-labeled tracer infusions followed by intermittent sampling of breath and blood. The rate of urea production was greater in infected children when compared to uninfected children (169 +/- 85 v 105 +/- 44 micromol urea x kg(-1) x h(-1), P <.02). For children with and without infection, the rates of leucine oxidation and urea production were directly correlated (r = 0.49 and r = 0.74, respectively; P <.01), but the slopes of the regression lines were different. In uninfected children the degree of wasting was correlated with the rates of urea production and leucine oxidation (r = 0.67 and r = 0.48, respectively; P <.05). These data suggest that the rates of leucine oxidation and urea production are both measures of nitrogen catabolism, that acute infection alters the relationship between the two, and that less nitrogen is lost as urea in children with more wasting.