In 12 female subjects participating in a 100-km run, the concentrations of cholesterol and triglycerides as well as those of HDL components (total cholesterol, cholesteryl ester, unesterified cholesterol, phosphatidyl choline, apolipoprotein A-I, and apolipoprotein A-II) were measured before and after the run. The concentrations of HDL unesterified cholesterol, HDL phosphatidyl choline, and HDL apolipoprotein A-I of the participants of the 100-km run were higher than the corresponding values of a control group of female company employees, whereas there was no difference in triglycerides, total cholesterol. HDL cholesterol, and HDL apolipoprotein A-II. Values of triglycerides and total cholesterol ascertained after the run were clearly decreased when compared to pre-exercise values (by 70% and 10%, respectively). Also, total protein content of the serum was marginally reduced after the run. Considering the reduction in protein level. HDL cholesterol and HDL apolipoprotein A-I values were raised after the run (P less than 0.01, P less than 0.05, respectively) while HDL phosphatidyl choline values decreased irrespective of the reduction in total protein (P less than 0.01). The quotients HDL cholesterol / total cholesterol as well as HDL cholesterol / HDL phosphatidyl choline were clearly higher after the run than before (P less than 0.01). The HDL apolipoprotein A-II level did not change post-exercise. These results indicate a change in the structure of HDL following a 100-km run in female athletes.