Aim To test the hypothesis that measures of aerobic fitness, body mass and indices of body composition will influence the metabolic and cardiovascular demands of simulated load-carriage tasks. Method Twenty-eight healthy male volunteers, following assessment of maximal oxygen uptake (VO(2)max) and body composition, walked on a treadmill at 4 kph (1.11 m/s) for 60 min on gradients of 0, 3, 6 and 9% whilst carrying backpack loads of 0, 20 and 40 kg. During the final 3 min of each 5-min exercise bout, indirect respiratory calorimetry and heart rate data were collected and the 'steady-state' metabolic VO2 and cardiovascular (heart rate) demands quantified. Results Absolute VO(2)max (ml/min) produced the strongest correlation (r=-0.64, P < 0.01) with the metabolic demand of heavy load-carriage (40 kg). The body composition index lean body mass/(fat mass + external load) produced a moderate correlation (r=-0.52, P < 0.01) with the metabolic demand of heavy load-carriage. The increases in metabolic and cardiovascular demands were greater when the load carried increased from 20 to 40 kg compared with 0 to 20 kg at all four gradients. A model incorporating anthropometric and physiological characteristics with gradient and load explains 89% of the variability in the metabolic demands of load-carriage compared with 82% using gradient and load alone. Conclusion The results show that indices of body composition as well as absolute aerobic power influence the relative metabolic demands of load-carriage. Application of these measurements would ensure selection criteria for load-carriage occupations are based on lean muscle mass rather than running speed.