Determine if the shortest sampling interval for laboratory variables used to estimate baseline severity of illness in pediatric critical care is equivalently sensitive across multiple sites without site-specific bias, while accounting for the vast majority of dysfunction compared with the standard 0- to 12-hour Pediatric Risk of Mortality III score. Prospective random patient selection. General/medical and cardiac/cardiovascular PICUs in eight hospitals. Patients younger than 18 years admitted to the PICU. None. A total of 376 patients were included. Measurements for Pediatric Risk of Mortality III laboratory variables (pH, PCO2, total CO2, PaO2, glucose, potassium, blood urea nitrogen, creatinine, total WBC count, platelet count, and prothrombin time/partial thromboplastin time) were recorded from 2 hours prior to PICU admission through 12 hours of PICU care except for data in the operating room. Decreasing the observation period from 0 to 12 hours post-PICU admission resulted in progressive decreases in the Pediatric Risk of Mortality III laboratory variables measured. However, allowing the observation period to start 2 hours prior to PICU admission to 4 hours reduced this loss to only 3.4%. Similar trends existed for each of the individual laboratory Pediatric Risk of Mortality III variables. There was a nearly identical distribution of laboratory Pediatric Risk of Mortality III points within the -2- to 4-hour period compared with the standard period. We did not detect any institutional bias using the -2- to 4-hour time period compared with the baseline. Prognostically important laboratory physiologic data collected within the interval from 2 hours prior to PICU to admission through 4 hours after admission account for the vast majority of dysfunction that these variables would contribute to Pediatric Risk of Mortality III scores. There was no institutional bias associated with this sampling period.