PURPOSE: This study examines risk factors for heat-related mortality due to hyperthermia in emergency department patients, a vulnerable population. METHODS: This matched case-control study used statewide, longitudinally linked emergency department (ED) data and death records from California. Cases comprised California residents (≥18 years) who presented to a state-licensed ED and died of hyperthermia during the study period (2009-2012). For each case, up to five ED patients were randomly selected as live controls and matched on sex and age. Patients' demographic characteristics and history of ED utilization for alcohol use, drug use, psychiatric disorders, heart-related conditions, chronic respiratory disease, neurodegenerative disorders, and cerebrovascular disease were assessed in relationship to hyperthermia mortality. RESULTS: Using multivariate conditional logistic regression models, hyperthermia mortality cases had higher odds of prior ED utilization for alcohol use (OR = 11.16, 95% CI = 3.87, 32.17) compared to controls. Cases were also more likely than controls to have Medicare insurance (OR = 5.80, 95% CI = 1.70, 15.15) or self-pay (OR = 5.39, 95% CI = 1.73, 16.79), at their most recent ED visit. CONCLUSIONS: ED patients presenting with alcohol problems may face increased risk of hyperthermia mortality. To help reduce heat-related mortality, EDs should consider interventions that target patients vulnerable to heat exposure.