To examine the associations of total plasma homocysteine (tHcy) with physical activity, cardiorespiratory fitness and fatness in children and adolescents, a cross-sectional study of 301 children (9-10 years old) and 379 adolescents (15-16 years old) was conducted. Physical activity was measured by accelerometry. Cardiorespiratory fitness was measured with a maximal ergometer bike test. Body fat was derived from the sum of five skinfold thicknesses. Genotyping for the methylenetetrahydrofolate reductase (MTHFR) 677C > T polymorphism was done by DNA sequencing. Fasting tHcy level was the outcome variable. Multiple regressions were used to determine the degree to which variance in tHcy was explained by physical activity, cardiorespiratory fitness and body fat, after controlling for potential confounders including MTHFR 677C > T genotype. tHcy levels were neither associated with any measure of level and pattern of physical activity nor with data on cardiorespiratory fitness, or body fat, in any age group after controlling for potential confounders including MTHFR 677C > T and even when subgroups 677TT and 677CC + CT were analysed separately. Mean values of tHcy were significantly higher in the TT subgroup compared with CC and CT subgroups in children (TT 7(.)4 mu mol/l, CC 6(.)3 mu mol/l, CT 6(.)6 mu mol/l, P < 0.001 and P=0019, respectively) and adolescents (TT 16(.)9 mu mol/l, CC 8(.)3 mu mol/l, CT 9(.)0 mol/l, both P < 0.001). The results suggest that physical activity, fitness and body fat are not associated with tHcy levels in children and adolescents, even after controlling for presence of the MTHFR 677C > T genotype, the main influence on tHcy levels in these subjects.