In familial hypercholesterolemia (FH), early coronary heart disease (CHD) is a complex trait that results from a large monogenic component of susceptibility due to elevated LDL cholesterol. This was demonstrated by observation of the high risk of early CHD in FH subjects compared with the general population. However, not all subjects with a LDLR gene mutation suffer with early CHD. Furthermore, studies in extended multigenerational families showed that even for this strong monogenic effect the environment could substantially modulate the age at death from CHD. Anecdotal examples of apparent modulation of atherosclerosis severity by lifestyle changes were also seen in other monogenic metabolic problems, such as hepatic lipase deficiency and Dunnigan-type familial partial lipodystrophy. Thus, even within apparently clear-cut rare monogenic metabolic diseases, such as FH, among carriers there can be a variability in the expression of important quantitative end points, such as early CHD. In some cases, the environment, including lifestyle factors, appears to play a key role in modulating the disease severity. This complexity could have implications for diagnosis and treatment.