The association of multiple organ system failure (MOSF) with mortality was investigated in 831 consecutive admissions to a pediatric ICU. The incidence of MOSF (at least two organ system failures, OSF) was 27%. Of the 62 nonsurvivors, 60 (97%) had MOSF. The mortality for patients with MOSF was 54%, compared to a mortality of 0.3% for patients without MOSF. Mortality increased directly with increasing number of OSF (p less than .0001). The mortality was 1% for one OSF, 11% for two OSF, 50% for three OSF, and 75% for four OSF. Comparison of these results with data from adult ICU patients indicates that the mortality and clinical course of MOSF in children is distinct from adults. MOSF is significantly associated with mortality in pediatric patients; however, it is not sufficiently discriminating to determine continuation or withdrawal of ICU support.