Heat exhaustion and heat stroke among U.S. active component service members, 2021-2025.

In 2025, the unadjusted incidence rates of heat stroke and heat exhaustion among U.S. active component service members were 39.4 and 170.0 cases per 100,000 person-years, respectively. The rate of heat stroke increased in 2024 and 2025, after declining for 3 years (2021-2023) at the beginning of the 5-year surveillance period. In contrast, the rate of heat exhaustion decreased in 2025, following a 4-year increase from 2021 through 2024. In 2025, male service members experienced higher rates of heat stroke, when compared to their female counterparts. Female service members and non-Hispanic Black service members experienced higher rates of heat exhaustion than their male counterparts and service members of other racial and ethnic groups, respectively. Consistent with prior annual reports, heat illness rates remained the highest among those younger than age 20 years, Marine Corps and Army members, and recruit trainees. To protect the force and increase readiness, military leaders and public health personnel can implement evidence-based prevention strategies, train service members to recognize the signs and symptoms of heat illness and take early action to counter the threat. The unadjusted incidence rate of heat stroke increased 6.9% from 2024 to 2025, for the second year in a row, while the unadjusted annual incidence rate of heat exhaustion fell by 9.1% in 2025, following a 4-year increase from 2021 through 2024. Locations where entry training are conducted for new Army, Air Force, Marine Corps, and Space Force personnel accounted for 40.3% of all heat illness diagnoses over the 5-year surveillance period. Training, specifically initial training upon service entry, remains a major risk factor for heat illness occurrence among U.S. active component service members.