Fifty-five healthy, sedentary, nonsmoking, and nonobese 24-to 26-year-old men who had low plasma concentrations of high density lipoprotein (HDL) cholesterol were selected for a study of the effect of short-term exercise on plasma llpld and lipoprotein concentrations. The participants were randomized Into two groups. Of these, 28 were assigned to a 9-week program of submaxlmal aerobic exercise three times weekly, and 27 were assigned to a nonexerclslng control group. Changes In physical fitness were assessed by Increments In estimated maximal oxygen consumption; this Increased by 15% In the exercise group (p<0.001) but remained unchanged In the control group. During the study, body weights and skinfold thicknesses of both groups remained essentially unchanged after 9 weeks. There was no significant difference between the trial groups In total cholesterol, HDL cholesterol, calculated low density lipoprotein cholesterol, or in the HDL 2 and HDL 3 subtractions. T he association between plasma concentration of high density lipoprotein (HDL) cholesterol and the incidence and severity of coronary heart disease has become well recognized in recent years.1 " 5 Programs of increased physical activity, particularly those based upon running or jogging, have attracted attention as being among the few potentially effective and physiologically desirable means of increasing plasma HDL cholesterol concentration. 678Several longitudinal studies have been conducted in initially sedentary, but healthy, individuals to measure the effect of increased physical activity on plasma lipoprotein concentrations. Some of these studies have led to the conclusion that increased exercise significantly elevates plasma HDL cholesterol concentrations, 9 " 15 whereas others have not confirmed this finding.16 '17> 18The aim of our study was to evaluate the efficacy of a 9-week program of moderate exercise to raise HDL cholesterol levels in young, nonobese, nonsmoking, sedentary men who initially had low plasma HDL cholesterol concentrations (=s40 mg/dl). Methods Study PopulationTotal plasma cholesterol, triglycerlde, and HDL cholesterol concentrations were measured in 8624 boys and girls aged 17 to 18 years. This was done in the framework of the Jerusalem LJpid Research Clinic Prevalence Study (LRC) Received May 12,1987; revision accepted December 4,1987. during the years 1976 to 1979 at the time of the teenagers' military induction health examination. Approximately 5000 of these participants had a second examination 3 to 4 years later at the end of their military service (Kark JD, unpublished data). Of the initial study population, 864 young people were students at the Hebrew University of Jerusalem during the years 1985 to 1986; 458 were men. The median plasma HDL cholesterol level of 17-to 18-year-old men in the LRC prevalence study was 43 mg/dl; upon discharge from military service, it was 42 mg/dl. To detect a 4 mg/dl increase in plasma HDL cholesterol, assuming an intraindividual standard deviation of 5 mg/dl, a B level of 0.10, and a one-...