Importance  Underweight and obesity represent opposite ends of the nutritional risk spectrum that negatively affect children’s health worldwide. Both conditions have been associated with increased mortality in pediatric patients receiving extracorporeal membrane oxygenation (ECMO) support.

Objective  To investigate the association of underweight and obesity status with pediatric ECMO outcomes using body mass index (BMI) z scores.

Design, Setting, and Participants  This retrospective cohort analysis used data from the Extracorporeal Life Support Organization (ELSO) database from January 1, 2019, to December 31, 2023. Participants included all pediatric patients from international ECMO centers aged 29 days to 18 years who received ECMO support during the study period. Data were analyzed from July 1, 2024 to June 30, 2025.

Exposure  BMI or weight for length.

Main Outcomes and Measures  Survival to hospital discharge.

Results  A total of 8885 children undergoing their first ECMO run during the study period were included. The median age was 1.9 years (IQR, 0.4-10.7 years), the median weight was 11 kg (IQR, 6-36 kg), and 4773 patients (53.7%) were male. Among the cohort, 6443 patients (72.5%) had a normal weight (z scores −2 to 2), 1458 (16.4%) had underweight (z score <–2), and 984 (11.1%) had obesity (z score >2). Overall hospital survival occurred in 5453 patients (61.2%) but was significantly lower in patients with underweight (849 [58.2%]) and obesity (565 [57.4%]) compared with normal weight (4020 [62.4%]; P < .001). In multivariable logistic regression, having underweight (odds ratio [OR], 1.34; 95% CI, 1.09-1.66; P = .005) and obesity (OR, 1.25; 95% CI, 1.00-1.56; P = .04) were associated with increased hospital mortality. This pattern persisted across subgroup analyses based on age and diagnosis leading to ECMO cannulation. Severe obesity (BMI z score >3) was not associated with mortality.

Conclusions and Relevance  In this cohort study of children receiving ECMO, 1 in 4 had an abnormal BMI, which was associated with increased hospital mortality. Prospective studies are warranted to further elucidate and define the association of nutritional status and clinical outcomes in this population.