BACKGROUND: An organ dysfunction (OD) scoring system for critically ill children is not yet available, and the method for developing such a system is not well defined. The aim of this study was to compare two developmental methods for assessing OD in critically ill children. METHODS: Consecutive admissions between January and May 1997 in three French and Canadian pediatric intensive care units (PICUs) were studied prospectively. Physiologic data were selected using a Delphi method; the most abnormal values during PICU stay were recorded. The outcome measure was the vital status at PICU discharge. Six organ systems were studied: hepatic, cardiovascular, renal, hematologic, respiratory, and neurologic. For each of the six organ systems, the PEdiatric Multiple OD (PEMOD) system included one variable and the PEdiatric Logistic OD (PELOD) system included several variables. Severity levels and relative weights of ODs were determined according to the mortality rate (PEMOD) or by logistic regression (PELOD). RESULTS: There were 594 admissions, including 51 deaths (9%). Severity levels and relative weights of ODs were: four levels graded from 1 to 4 for the PEMOD system and three levels with scores of 1, 10, and 20 for PELOD system. For both systems, calibrations were good (p = 0.23 and p = 0.44 respectively). The PELOD system was more discriminant than the PEMOD system (areas under the ROC curves 0.98 and 0.92, respectively, p < 10(-5)). Moreover, with the PEMOD system, four ODs did not contribute significantly to the prediction of PICU outcome. CONCLUSIONS: The PELOD system was more discriminant and had the advantage of taking into account both the relative severities among ODs and the degree of severity of each OD.