IMPORTANCE Therapeutic hypothermia reduces risk of death and disability in infants with moderate to severe hypoxic ischemic encephalopathy (HIE). Randomized clinical trials of therapeutic hypothermia to date have not included infants with mild HIE because of a perceived good prognosis.OBJECTIVE To test the hypothesis that children with mild HIE have worse neurodevelopmental outcomes than their healthy peers.DESIGN, SETTING, AND PARTICIPANTS Analysis of pooled data from 4 prospective cohort studies in Cork, Ireland, and Stockholm, Sweden, between January 2007 and August 2015. The dates of data analysis were September 2017 to June 2019. Follow-up was performed at age 18 to 42 months. In this multicenter cohort study, all children were born or treated at the tertiary centers of Cork University Maternity Hospital, Cork, Ireland, or Karolinska University Hospital, Stockholm, Sweden. In all, 690 children were eligible for this study.EXPOSURES At discharge, all children were categorized into the following 5 groups using a modified Sarnat score: healthy controls, perinatal asphyxia (PA) without HIE, mild HIE, moderate HIE, and severe HIE.MAIN OUTCOMES AND MEASURES Cognitive, language, and motor development were assessed with the Bayley Scales of Infant and Toddler Development, Third Edition (BSITD-III). The BSITD-III scores are standardized to a mean (SD) of 100 (15), with lower scores indicating risk of developmental delay.RESULTS Of the 690 children eligible for this study, 2-year follow-up data were available in 471 (mean [SD] age at follow-up, 25.6 [5.7] months; 54.8% male), including 152 controls, 185 children with PA without HIE, and 134 children with HIE, of whom 14 had died. Infants with mild HIE (n=55) had lower cognitive composite scores compared with controls, with a mean (SD) of 97.6 (11.9) vs 103.6 (14.6); the crude mean difference was -6.0 (95% CI, -9.9 to -2.1), and the adjusted mean difference was -5.2 (95% CI, -9.1 to -1.3). There was no significant difference in the mean cognitive composite scores between untreated children (n=47) with mild HIE and surviving children with moderate HIE (n=53) treated with therapeutic hypothermia, with a crude mean difference for mild vs moderate of -2.2 (95% CI, -8.1 to 3.7).CONCLUSIONS AND RELEVANCE This study's findings suggest that, at age 2 years, the cognitive composite scores of children with a history of mild HIE may be lower than those of a contemporaneous control group and may not be significantly different from those of survivors of moderate HIE treated with therapeutic hypothermia.