The 'oxidation hypothesis' states that oxidative modification of low-density lipoprotein (LDL) is important in the pathogenesis of the atherosclerotic lesion. Physical exercise has been reported to reduce clinical manifestations of atherosclerosis. We compared the effects of aerobic physical exercise history on oxidation characteristics of plasma LDL at rest in non-smoking male veteran endurance athletes (n= 15) and control subjects (n= 16). As an indicator of LDL oxidation we used a method which provides an estimate of LDL diene conjugation in vivo. The athletes had lower plasma LDL cholesterol diene conjugation (P=0.003). They also had lower cholesterol, lower LDL and higher high-density lipoprotein (HDL) cholesterol. Intensive physical exercise energy expenditure correlated inversely with LDL diene conjugation (r=-0.41, P= 0.021). We conclude that participation in intensive physical exercise training is associated with reduced plasma LDL fraction oxidation as reflected by LDL diene conjugation. This finding may have relevance for studies aiming to explain the variation in occurrence of ischemic heart disease.