The purpose of this study was to investigate by direct measurement the cross-sectional relationship between accelerometer-measured physical activity and peak oxygen uptake (VO(2peak): ml x min(-1) x kg(-1)), in a population-based cohort of young children, since such data are scarce. The study included 468 children (246 boys, 222 girls) aged 6.7 + or - 0.4 years, recruited from a population-based cohort. Peak oxygen uptake was measured by indirect calorimetry during a maximal treadmill exercise test. Physical activity was assessed by accelerometers over a 4-day period. Minutes of sedentary, light, moderate, moderate-to-vigorous, and vigorous activity per day were calculated. Mean counts per minute were considered to reflect total physical activity. Pearson correlation coefficients indicated a weak relationship between daily physical activity variables and VO(2peak) in boys (r = 0.15-0.28, P < 0.05), with the exception of time in sedentary and light activity, which was not related to VO(2peak). None of the daily physical activity variables were related to VO(2peak) in girls, with the exception of a very weak relationship for moderate activity (r = 0.14, P < 0.05). Multiple regression analyses indicated that the various physical activity variables explained between 2 and 8% of the variance in VO(2peak) in boys. In this population-based cohort, most daily activity variables were positively related to aerobic fitness in boys, whereas less clear relationships were observed in girls. Our finding that physical activity was only uniformly related to aerobic fitness in boys partly contradicts previous studies in older children and adolescents.