Recent studies of patients with coronary heart disease (CHD) at baseline have shown that their cholesterol levels are much less predictive of subsequent mortality than in populations free of CHD (FCHD). One previously suggested explanation of this attenuation is that the impact of hyperlipidemia on atherosclerosis or of atherosclerosis on mortality is reduced for post-myocardial infarction patients. In this paper it is shown that an alternative explanation is selection of CHD populations from FCHD populations for higher atherosclerosis levels. Data from all known follow-up studies on patients with baseline coronary angiograms are assembled to yield relations between cholesterol, atherosclerosis and mortality in CHD and FCHD populations. These data show that the selection hypothesis is not only logically possible but is also consistent with presently available epidemiologic information on relations between these three variables. An ethically impracticable large prospective study of a FCHD population with baseline angiograms might, however, be needed to choose definitively between the selection and reduced impact hypotheses.