Urinary urea nitrogen (UUN) excretion as an index of protein catabolism was assayed in 32 children (2m to 15y, median 6y)(50% mechanically ventilated) during an ICU course of 1 to 10 days (median 3d). Mean daily UUN excretion was 171 ± 89 mg/kg (4.38 ± 2.22 gm/m2), with greater excretion within 24 hours of ICU admission than subsequently. Average daily nitrogen balance per child was - 146 ± 82 mg/kg (-3.73 ± 2.04 gm/m2), and was independent of caloric intake. Average daily UUN excretion per child was well described by regression equations for weight (mg = 219.76(kg) − 1.74(kg)2, r2 = 0.908), height (mg = 4.07(cm) + 0.25(cm)2, r2 = 0.917), and meter squared body surface area (mg = 4421.5(BSA), r2=0.903). Excretion data in mechanically ventilated versus spontaneously breathing children, and in 4 diagnostic subgroups (sepsis 6, Reye syndrome 7, elective surgery 7, and miscellaneous 12) was evenly distributed about regression lines for length, weight, and body surface area. Increased UUN excretion accompanied isoproterenol infusion and prednisone administration. Decreased excretion accompanied insulin infusion and high blood levels of barbiturates. This study documents the magnitude and time course of protein catabolism in critically ill children and suggests rarely considered drug effects. It confirms progressive protein depletion at per kg rates of UUN excretion comparable to critically ill adults with wide individual variability but little variation between diagnostic subgroups.