Body cooling is recommended for patients with
heat stroke and
heat exhaustion. However, differences in the outcomes of patients who do or do not receive active cooling
therapy have not been determined. The best available evidence supporting active cooling is based on a case series without comparison groups; thus, the effectiveness of this method in improving patient prognoses cannot be appropriately quantified. Therefore, we compared the outcomes of
heat stroke patients receiving active cooling with those of patients receiving
rehydration-only
therapy. This prospective observational multicenter registry-based study of
heat stroke and
heat exhaustion patients was conducted in Japan from 2010 to 2019. The patients were stratified into the "severe" group or the "mild-to-moderate" group, per clinical findings on admission. After conducting multivariate logistic regression analyses, we compared the prognoses between patients who received "active cooling +
rehydration" and patients who received "
rehydration only," with in-hospital death as the endpoint. Sex, age, onset situation (i.e., exertional or non-exertional), core body temperature, liver damage, renal dysfunction, and
disseminated intravascular coagulation were considered potential covariates. Among those who received active cooling and
rehydration-only
therapy, the in-hospital mortality rates were 21.5% and 35.5%, respectively, for severe patients (n = 231) and 3.9% and 5.7%, respectively, for mild-to-moderate patients (n = 578).
Rehydration-only
therapy was associated with a higher in-hospital mortality in patients with severe
heat illness (adjusted odds ratio [aOR], 3.29; 95% confidence interval [CI], 1.21-8.90), whereas the cooling methods were not associated with lower in-hospital mortality in patients with mild-to-moderate
heat illness (aOR, 2.22; 95% CI, 0.92-5.84). Active cooling was associated with lower in-hospital mortality only in the severe group. Our results indicated that active cooling should be recommended as an adjunct to
rehydration-only
therapy for patients with severe
heat illness.