The authors studied the association of the type, amount, and intensity of physical activity with serum lipids in 2,492 randomly selected eastern Finnish men aged 42-60 years during 1984-1989, controlling for the major confounding factors. High density lipoprotein cholesterol (HDL cholesterol) and HDL2 cholesterol were associated positively and triglycerides inversely with total, occupational, and leisure time activity in a multivariate regression model (p < 0.001). HDL3 cholesterol was associated positively only with total activity (p < 0.001). The adjusted relative differences in HDL cholesterol, HDL2 cholesterol, HDL3 cholesterol, and triglycerides between extreme total (occupational, leisure time) activity quartiles were 10.7% (9.0%, 5.7%), 12.2% (10.3%, 9.5%), 5.9% (2.9%, 0%), and 22.7% (9.2%, 10.4%), respectively. HDL cholesterol and HDL2 cholesterol were the highest and triglycerides the lowest at a conditioning activity intensity of more than 6 metabolic units (p < 0.001). The strongest associations were found for the 24-Hour Recording, which suggests short-term effects of physical activity on serum lipids. The beneficial effects of physical activity on serum lipids appear to be mediated partially by a reduced serum insulin concentration and decreased body adiposity. These data provide further confirmation for the concept that both regular or at least moderate intensity leisure time activity and occupational physical activity have a favorable impact on serum lipids.