Cross-sectional studies have demonstrated in the past that endurance-trained persons are characterised by a less atherogenic lipid profile than their sedentary counterparts: the former have clearly higher HDL-cholesterol and lower serum triglyceride concentrations than the latter, and also their LDL-cholesterol and total cholesterol concentrations are slightly lower. In a longitudinal intervention study in previously sedentary men, the effect of moderate short term physical training on the serum lipid profile was investigated. 30 healthy male volunteers were trained for 16 weeks, 3 hours weekly. After training their physical working capacity was increased on average by 29%. This was accompanied by an increment in the HDL-cholesterol fraction of 26%; the relative increase was greater for HDL2-(+32%) than for HDL3-cholesterol(+24%). VLDL-cholesterol was reduced by 21% at the end of the training period. On average, plasma total cholesterol, total triglycerides and LDL-cholesterol were not significantly changed at the end of the training period. Negative associations were, however, found between the training-induced increase in exercise capacity and the concomitant changes in plasma triglycerides and LDL-cholesterol. In conclusion, these longitudinal observations show that short term moderate endurance training improves the serum lipid profile in previously sedentary men.