In this study we tested the hypothesis that performing 1 h of regular light exercise in a heat chamber (HEAT; 37.8 ± 0.5 • C; 65.4 ± 1.8% humidity) 5 times week −1 for a total of 5 weeks increases haemoglobin mass (Hb mass) and exercise performance in elite cyclists (V O 2 max = 76.2 ± 7.6 ml min −1 kg −1). Twenty-three male volunteers were assigned to HEAT (n = 11) or CON (n = 12; 15.5 ± 0.1 • C; 25.1 ± 0.0% humidity) training groups. Hb mass was determined before and after the intervention period in conjunction with an extensive exercise test protocol (conducted at 16-19 • C). HEAT increased (P < 0.05) Hb mass by 42 g from 893 ± 78 to 935 ± 108 g whereas Hb mass remained unchanged (+6 g) in CON. Furthermore, statistical analysis revealed a time-group interaction (P < 0.05). The greater increase in Hb mass in HEAT, however, did not manifest in a greater increase inV O 2 max (225 ± 274 ml min −1 in HEAT and 161 ± 202 ml min −1 in CON). While HEAT reduced (P < 0.05) lactate levels during some of the submaximal exercise tests, there was no statistical difference between other performance parameters. There were, however, small to intermediate effect sizes favouring HEAT for lactate threshold power output (2.8 ± 3.9 vs. −0.4 ± 5.1% change, effect size (ES) = 0.34), gross economy in the fatigued state (0.19 ± 0.42 vs. −0.12 ± 0.49%-point change, ES = 0.52) and 15 min mean power (6.9 ± 8.4 vs. 3.4 ± 5.1% increase, ES = 0.22). This study demonstrates an increase in Hb mass and small to intermediate effect sizes on exercise variables in elite cyclists following a 5-week heat training intervention. K E Y W O R D S blood volume, exercise, hot environment 1 INTRODUCTION A high maximal oxygen uptake (V O 2 max) is essential for successful exercise performance in endurance sport disciplines. WhileV O 2 max is the product of the Fick equation in which both O 2 transport to the exercising skeletal muscles and O 2 extraction by the skeletal muscle contribute, the biggest difference between individuals with This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.