Thirty-four healthy, untrained middle-aged males (mean = 58.2 years) volunteered for a longitudinal study investigating alterations in lipoprotein profiles consequent to 20 weeks of aerobic training. Twenty randomly selected experimental subjects (E) participated in a walk/run training program 3 d/wk, 48 min/d, at an intensity equal to 65% to 85% of their heart rate reserve, while 14 control subjects (C) remained in an untrained state. Blood samples were obtained in the morning after a 14-hour fast. Plasma lipoproteins were separated by ultracentrifugation and agarose-column chromatography into very low (VLDL), low (LDL), and high (HDL) density lipoprotein fractions. A training effect was evidenced in E by significant increases (P less than 0.001) in absolute VO2 max (15.2%) and relative VO2 max (16.8%). Dietary composition was monitored pretraining and posttraining with no significant change seen in either E or C. Alterations in the lipoprotein profile of E included a significant increase in HDL-cholesterol (16.6%) and significant reductions in VLDL-cholesterol (72.1%), LDL-triglyceride (26.5%), VLDL-protein (33.5%), and LDL-protein (18.7%). Furthermore, no significant changes in the level of HDL-protein and apolipoprotein AI in the HDL fraction were observed in either E or C. These data suggest that aerobic training can significantly increase HDL-cholesterol and alter lipoprotein profiles, which may subsequently reduce the risk of coronary artery disease.