Concentrations of serum lipoprotein lipids and apolipoproteins A-I, A-II and B were determined 3-6 months after myocardial infarction in 116 males below the age of 45 and in 116 age-matched controls. Among single variables the sum of cholesterol concentration in VLDL and LDL divided by the HDL cholesterol level was the best discriminator between patients and controls. The concentrations of serum triglycerides, apolipoprotein B, VLDL triglycerides and cholesterol, serum cholesterol, HDL cholesterol and LDL triglycerides, in that order, were better discriminators than was LDL cholesterol level. Among variables reflecting HDL concentration and composition HDL cholesterol was the best discriminator followed by HDL2 cholesterol, apolipoprotein A-I and the HDL cholesterol/apolipoprotein A-I ratio. Multivariate analysis indicated independent significance of elevated VLDL lipid and LDL cholesterol concentrations, and a decreased HDL cholesterol concentration, in relation to MI. The present data suggest that a disturbed triglyceride metabolism, in addition to elevated LDL and decreased HDL cholesterol levels, has an independent and pathogenetic significance for MI at a young age.