Serum concentrations of lipoprotein mass by flotation rate were measured in 12 long-distance runners and 64 sedentary men by analytic ultracentrifugation. The runners had significantly lower serum mass concentrations of the smaller, denser low-density lipoprotein particles of flotation rates Sf 0-7 (including the LDL-II, LDL-III, and LDL-IV subspecies), very-low-density lipoprotein (VLDL) particles of Sf 20-400, and high-density lipoprotein (HDL) particles of flotation rates F1.20 0-1.5 (predominantly the HDL3 subspecies), and higher serum mass concentrations of HDL particles with flotation rates between F1.20 2.0-9.0 (including HDL2a and HDL2b and less dense particles belonging to HDL3) than did sedentary men. Lipoprotein lipase activity was higher, and hepatic lipase activity was lower in runners than in the sedentary men. Thus, the effects of endurance exercise training to lower LDL may be specific to the smaller, denser LDL particle region. Similarities in the lipoprotein mass profiles of the runners and the low-risk profiles of sedentary, middle-aged women suggest the effects of common metabolic factors possibly leading to reduced risk of coronary artery disease.