Triglycerides (TG), total cholesterol (TC) and HDL-cholesterol (HDL-C) were determined in 48 long-distance runners (group I) and in 34 sedentary males of similar age-range and weight serving as controls (group II). Blood samples were taken before and after racing in runners and at rest in controls, but comparisons were made between baseline values only. Taken globally, group I subjects had significantly higher HDL-C levels (p less than 0.01) and lower TC/HDL-C ratios (p less than 0.05) than group II subjects. However, considerable changes were found within and between the groups when subjects were divided according to age. Thus, there was no significant difference in blood lipids between long-distance runners and controls in the 20-34 years subgroups, whereas in the 35-55 years subgroups HDL-C values tended to decrease in controls but remained high in long-distance runners, the difference being highly significant (p less than 0.001). Consequently, there also was a significant difference (P less than 0.01) between group 1 and group II subjects in TC: HDL-C ratios. The increase in TG with age was much greater in controls than in runners. It is suggested that high HDL-C levels might be one of the mechanisms though which intensive physical training exerts a protective effect against coronary disease.