OBJECTIVE: To assess the prevalence of serious infections and mortality among infants ≤90 days of age presenting to the emergency department with hypothermia. STUDY DESIGN: We performed a cross-sectional cohort study of infants ≤90 days presenting to any of 40 EDs in the Pediatric Health Information Systems between January 1, 2009, and December 31, 2018. Infants with an International Classification of Diseases, ninth or tenth edition, admission/discharge diagnosis code of hypothermia were included. We determined the prevalence of serious bacterial infection (urinary tract infection, bacteremia, and/or bacterial meningitis), pneumonia, herpes simplex virus (HSV) infection, and emergency department/hospital mortality. RESULTS: We included 3565 infants (1633 male [50.9%] and 3225 ≤30 days of age [90.5%]). Most (65.0%) presented in the first week of life. There were 389 infants (10.8%) with a complex chronic condition. The prevalence of serious bacterial infection was 8.0% (n = 284), including 2.4% (n = 87) with urinary tract infection, 5.6% (n = 199) with bacteremia, and 0.3% (n = 11) with bacterial meningitis. There were 7 patients (0.2%) with neonatal HSV and 9 (0.3%) with pneumonia; 0.2% (n = 6) died. The presence of a complex chronic condition was associated with the presence of serious bacterial infection (P < .001) and was present in 3 of 6 patients who died. In a sensitivity analysis including patients with any diagnosis code of hypothermia (n = 8122), 14.9% had serious bacterial infection, 0.6% had HSV, and 3.3% had pneumonia; 2.0% died. CONCLUSIONS: Of infants with hypothermia ≤90 days of age, 8.3% had serious bacterial infections or HSV. Compared with literature from febrile infants, hypothermia is associated with a high mortality rate. Complex chronic conditions were particularly associated with poor outcomes. Additional research is required to risk stratify young infants with hypothermia.