BACKGROUND: Multiple organ dysfunction syndrome is more frequent than death in paediatric intensive care units. Estimation of the severity of this syndrome could be a useful additional outcome measure in clinical trials in such units. We aimed to validate the paediatric logistic organ dysfunction (PELOD) score and estimate its validity when recorded daily (dPELOD). METHODS: We did a prospective, observational, multicentre cohort study in seven multidisciplinary, tertiary-care paediatric intensive care units of university-affiliated hospitals (two French, three Canadian, and two Swiss). We included 1806 consecutive patients (median age 24 months; IQR 5-90). PELOD score includes six organ dysfunctions and 12 variables and was recorded daily. For each variable, the most abnormal value each day and during the whole stay were used in calculating the dPELOD and PELOD scores, respectively. Outcome was vital status at discharge. We used Hosmer-Lemeshow goodness-of-fit tests to evaluate calibration and areas under receiver operating characteristic curve (AUC) to estimate discrimination. FINDINGS: 370 (21%) patients had no organ dysfunction, 471 (26%) had one, 457 (25%) had two, and 508 (28%) had three or more. Case fatality rate was 6.4% (115 deaths). PELOD score was significantly higher in non-survivors (mean 31.0 [SE 1.2]) than survivors (9.4 [0.2]; p<0.0001). Calibration (p=0.54) and discrimination (AUC=0.91, SE=0.01) of PELOD and dPELOD (p> or =0.39; AUC> or =0.79) scores were good. INTERPRETATION: PELOD and dPELOD scores are valid outcome measures of the severity of multiple organ dysfunction syndrome in paediatric intensive care units; their use should significantly reduce the sample size required to complete clinical trials in critically ill children.