Leptin is a hormone that reflects the body fat content. It was reported that serum leptin levels were decreased in highly endurance-trained sportsmen in comparison with control non-sporting subjects. The aim of our work was to study the relation of serum leptin to blood viscosity and selected spiroergometric parameters of endurance capacity in a group of top rugby players and top race walkers. We have found that both body fat content and serum leptin levels were significantly lower in race walkers than in rugby players (9.68+/-3.65 vs 15.95+/-3.15% and 2.84+/-1.1 vs 3.89+/-1.09 ng x ml(-1) respectively, p<0.05). The positive correlation of serum leptin levels with body fat in both groups. The level of endurance training status was significantly higher in the race walkers group. Serum leptin levels significantly negatively correlated oxygen uptake per body and pulse oxygen per body weight only in rugby players but not in race walkers. Partial correlation test after adjusting for the effect of body fat content showed that leptin itself is not an independent predictor of endurance trainability in this group. Serum leptin levels correlated positively with blood viscosity only in race walkers, but not in the rugby players group. We conclude that serum leptin levels in top sportsmen parallel the changes in body fat content and are not an independent predictor of endurance training of these subjects.