Athletes may incorporate periods of training at altitude (normobaric and hypobaric) in their annual training programmes. Training intensities are often controlled or prescribed using a percentage of an athlete’s maximal heart rate (HRmax). However, the direct measurement of HRmax may not be available, leading athletes and coaches to estimate it using age-related predictive equations with the two most commonly used being; 1: 220-age (1) and 2: 208-0.7*age (2). Research examining the response of HRmax to acute hypoxia while inconclusive; suggest HRmax may be altered suggesting that the use of predictive equations in hypoxia may not be valid. The purpose of the present study was to examine the accuracy of these two equations for predicting HRmax in adults exposed to acute hypoxia. Fifteen healthy volunteers (7 women; age 22±2 years; height 176±10 cm; body mass 73±14 kg; body mass index 23±3 kg.m2; VO2max 45±7 ml.kg-1.min-1) participated in the study. Participants performed incremental cycle ergometer exercise tests (women: 20 W.min-1, men: 25 W.min-1) to exhaustion at sea level (SL) and in normobaric hypoxia replicating four altitudes (1000, 2000, 3000 and 4000 m) in a single-blind manner. Heart rate (Polar F1, Polar Electro) was measured continuously and the peak 10s rate taken as HRmax, oxygen saturation (SPO2) was also recorded. Data were analysed with repeated measures ANOVA and paired t-tests. Measured HRmax was similar in all conditions including SL (182±13, 178±11, 177±9, 178±9, 176±11 b.min-1) despite a reduction in maximal SPO2 with increasing altitude (95±5; 95±2; 92±2; 88±3; 82±4%; P<0.05). The HRmax predicted with both equations (Equation 1; 198±2 b.min-1, Equation 2; 192±1 b.min-1) was higher than measured for all conditions except SL (Equation 1: 9, 11, 12, 11, 13%; Equation 2: 6, 8, 9, 8, 10%; P<0.05). Commonly used age-related predictive equations overestimated HRmax, which could lead to the inaccurate prescription of training intensities. Such overestimations at altitude may result in a training programme that is unachievable for the athlete, effecting their motivation and exposing them to an increased risk of overtraining or acute mountain sickness (3). It is recommended that a direct altitude or hypoxia specific measure of HRmax is used for prescribing training intensities at altitude in the absence of a valid predictive equation.