This cross-sectional study examined the association between blood pressure and physical fitness in 1,369 children (81.5% of the eligible population) aged 8-10 years participating in the first phase of a primary intervention study with physical training. An inverse correlation between BP and physical fitness was demonstrated, and significant differences have been found between quartiles of distribution of physical fitness in girls with regard to systolic BP and in boys and girls with regard to diastolic BP respectively. Important determinants of physical fitness seemed to be dependent on confounding variables representing body mass index, height, heart rate and age. Our observations suggest that the contribution of confounding variables may explain the relationship between BP and physical fitness in childhood.