The serum concentrations of total cholesterol and triglycerides as well as the cholesterol, phosphatidyl choline, and apolipoprotein components of HDL were tested in 19 participants immediately before and immediately after a 100-mile run. After the run, taking into account any alterations in total protein, the following changes were observed: a decrease in total cholesterol (p less than 0.001), as well as in triglycerides (p less than 0.05), and an increase in HDL cholesterol (p less than 0.05), and HDL phosphatidyl choline (p less than 0.01). The concentrations of HDL apolipoprotein A-I and HDL apolipoprotein A-II were not affected. The results indicate a change in composition of HDL following extreme prolonged physical exercise in women.