The familial lipoprotein disorder type III hyperlipoproteinemia (HPL) carries a marked increase in the risk of accelerated and premature atherosclerosis, but there is considerable variation among affected individuals in susceptibility to cardiovascular disease (CVD). We studied the influence of independent risk factors for atherosclerosis in 67 patients with clinically overt type III HPL and homozygosity for apolipoprotein (apo) E2. Among the different risk factors (lipid and lipoprotein levels, age, sex, body mass index, smoking status, hypertension, and diabetes mellitus) there was only a statistically significant difference in age between 25 patients with atherosclerosis and 42 patients without atherosclerosis. Serum lipoprotein (a), [Lp, (a)], levels were 30.6% higher in the atherosclerosis group, but this was not statistically significant. We conclude that (in contrast to familial hypercholesterolemia) elevated Lp (a) concentrations may not be regarded as a component of the clinical syndrome of type III HPL.