The effects of regular aerobic exercise on blood concentrations on high-density lipoprotein cholesterol and prostacyclin were studied in 18 healthy sedentary boys who exercised on a bicycle ergometer for 1 h, four times per week, for 2 months at 75% of their physical working capacity. The results were compared with those of 10 control boys who did not participate in any specific program. The exercise group revealed a significantly higher mean high-density lipoprotein cholesterol and 6-keto-PGF1a concentration at the end of the training program (p < 0.005 and p < 0.001, respectively). These results suggest that regular aerobic exercise has beneficial effects upon the high-density lipoprotein cholesterol and 6-keto-PGF1a in healthy young boys and may be beneficial in the long term, by preventing atherosclerosis of coronary artery disease in adulthood.