Recent studies demonstrated associations between occlusive vascular disease and hyperhomocysteinemia of both genetic and nutritional origin. In the present study we analyzed plasma samples from the 20th biannual examination of the Framingham Heart Study cohort to determine distribution of plasma homocysteine concentrations with emphasis on relationships to B vitamins and prevalence of carotid artery stenosis. Results showed that homocysteine exhibited strong inverse association with plasma folate and weaker associations with plasma vitamin B-12 and pyridoxal-5'-phosphate (PLP). Homocysteine was also inversely associated with intakes of folate and vitamin B-6, but not vitamin B-12. Prevalence of high homocysteine (>14 micromol/l) was 29.3% in this cohort, and inadequate plasma concentrations of one or more B vitamins appear to contribute to 67% of the cases of high homocysteine. Prevalence of stenosis > or = 25% was 43% in men and 34% in women with an odds ratio of 2.0 for individuals in the highest homocysteine quartile (> or = 14.4 micromol/l) compared with those in the lowest quartile (< or = 9.1 micromol/l), after adjustment for sex, age, high density lipoprotein cholesterol, systolic blood pressure and cigarette smoking (Ptrend < 0.001). Plasma concentrations of folate and pyridoxal-5'-phosphate and folate intake were inversely associated with extracranial carotid stenosis after adjustment for age, sex and other risk factors.