There is increasing anecdotal evidence that simple occupational tests of aerobic fitness impose a systematic bias against heavier personnel when predicting fitness for load‐carrying tasks. This study tested the hypothesis that simple field tests of aerobic fitness are not good predictors of load‐carrying performance and that personnel with greater body mass are more able to perform occupationally relevant load‐carrying tasks. Twelve healthy male volunteers ran on a level treadmill at 9.5 km/h for 4 min, with (T18) and without (T0) an external backpack load of 18 kg. During each exercise period, steady‐state oxygen uptake (VO2) was assessed. On a subsequent occasion (at least 7 days later), 11 of the subjects ran to exhaustion at 9.5 km/h whilst carrying the 18 kg external load (ETT18). There was a strong inverse linear relationship between relative VO2 and body mass (r = −0.87, P < 0.01) and between VO2 and lean body mass (r = −0.74, P < 0.01) during the T18 trials. Furthermore, there was a moderately strong relationship between exercise time (ETT18) and body mass (r = 0.69, P < 0.05) and between exercise time and lean body mass (r = 0.71, P < 0.05). There was no relationship between exercise tolerance time and VO2 (r = 0.12). The results show that fitness tests that determine aerobic power in units relative to body mass (e.g. timed distance run) incur a systematic bias against heavier personnel. Such tests are therefore inappropriate when predicting the ability of personnel to work in occupations that encompass load‐carrying tasks.