IMPORTANCE: Functional status assessment methods are important as outcome measures for pediatric critical care studies. OBJECTIVE: To investigate the relationships between the 2 functional status assessment methods appropriate for large-sample studies, the Functional Status Scale (FSS) and the Pediatric Overall Performance Category and Pediatric Cerebral Performance Category (POPC/PCPC) scales. DESIGN, SETTING, AND PARTICIPANTS: Prospective cohort study with random patient selection at 7 sites and 8 children's hospitals with general/medical and cardiac/cardiovascular pediatric intensive care units (PICUs) in the Collaborative Pediatric Critical Care Research Network. Participants included all PICU patients younger than 18 years. MAIN OUTCOMES AND MEASURES: Functional Status Scale and POPC/PCPC scores determined at PICU admission (baseline) and PICU discharge.We investigated the association between the baseline and PICU discharge POPC/PCPC scores and the baseline and PICU discharge FSS scores, the dispersion of FSS scores within each of the POPC/PCPC ratings, and the relationship between the FSS neurologic components (FSS-CNS) and the PCPC. RESULTS: We included 5017 patients.We found a significant (P < .001) difference between FSS scores in each POPC or PCPC interval, with an FSS score increase with each worsening POPC/PCPC rating. The FSS scores for the good and mild disability POPC/PCPC ratings were similar and increased by 2 to 3 points for the POPC/PCPC change from mild to moderate disability, 5 to 6 points for moderate to severe disability, and 8 to 9 points for severe disability to vegetative state or coma. The dispersion of FSS scores within each POPC and PCPC rating was substantial and increased with worsening POPC and PCPC scores.We also found a significant (P < .001) difference between the FSS-CNS scores between each of the PCPC ratings with increases in the FSS-CNS score for each higher PCPC rating. CONCLUSIONS AND RELEVANCE: The FSS and POPC/PCPC system are closely associated. Increases in FSS scores occur with each higher POPC and PCPC rating and with greater magnitudes of change as the dysfunction severity increases. However, the dispersion of the FSS scores indicated a lack of precision in the POPC/PCPC system when compared with the more objective and granular FSS. The relationship between the PCPC and the FSS-CNS paralleled the relationship between the FSS and POPC/PCPC system.