In this ancillary study to the prospective Cardiovascular Health Study (CHS), a case-cohort design was used to test hypotheses that the risk of myocardial infarction and/or stoke was associated with elevated plasma levels of homocysteine, and low plasma levels of folate, vitamins B12 and B6. Further, a determination was made whether elevated plasma levels of homocysteine and Lp(a) interacted to increase substantially the risk of myocardial infarction and/or stroke above that due to either factor alone. The sub-cohort was used to study the relationship between the factors under study and progression of sub-clinical atherosclerosis. For each case and sub-cohort member, an aliquot of fasting plasma drawn at baseline was analyzed for homocysteine, folate B12, and B6 concentrations. [Values of plasma Lp(a) were determined at baseline.] Results of these assays were combined with other CHS data to address the hypothesis that the risk of myocardial infarction and/or stroke was associated with elevated plasma levels of homocysteine, and low plasma levels of folate, vitamins B12 and B6.