Purpose: The present case report aimed to investigate the effects of exercise training in temperate ambient conditions while wearing a heat suit on hemoglobin mass (Hb_mass). 

Methods: As part of their training regimens, 5 national-team members of endurance sports (3 males) performed ∼5 weekly heat suit exercise training sessions each lasting 50 minutes for a duration of ∼8 weeks. Two other male athletes acted as controls. After the initial 8-week period, 3 of the athletes continued for 2 to 4 months with ∼3 weekly heat sessions in an attempt to maintain acquired adaptations at a lower cost. Hb_mass was assessed in duplicate before and after intervention and maintenance period based on automated carbon monoxide rebreathing. 

Results: Heat suit exercise training increased rectal temperature to a median value of 38.7°C (range 38.6°C–39.0°C), and during the initial ∼8 weeks of heat suit training, there was a median increase of 5% (range 1.4%–12.9%) in Hb_mass, while the changes in the 2 control athletes were a decrease of 1.7% and an increase of 3.2%, respectively. Furthermore, during the maintenance period, the 3 athletes who continued with a reduced number of heat suit sessions experienced a change of 0.7%, 2.8%, and −1.1%, indicating that it is possible to maintain initial increases in Hb_mass despite reducing the weekly number of heat suit sessions. 

Conclusions: The present case report illustrates that heat suit exercise training acutely raises rectal temperature and that following 8 weeks of such training Hb_mass may increase in elite endurance athletes.