Abnormalities in homocysteine metabolism have been suggested as risk factors for stroke and myocardial infarction. In retrospective studies, elevated levels of total plasma homocysteine (tHcy) and/or methylenetetrahydrofolate reductase (MTHFR) 677C>T polymorphism have indicated an increase in risk. However, the fewer prospective studies have not been as conclusive. To further explore this, tHcy was studied in four prospective settings. The first was a prospective nested case-referent cohort within the Västerbotten Intervention Program (VIP) and WHO MONICA project on 312 ischemic and 60 haemorrhagic first-ever strokes. The aim was to study tHcy and its main genetic determinant MTHFR. Risk for haemorrhagic stroke increased exponentially through tHcy quartiles, independent of hypertension and BMI, and increased for MTHFR 677 CT and TT. MTHFR 1298A>C appeared to be protective. In multivariate models, after adjustment for tHcy, BMI and hypertension, both tHcy and MTHFR remained as independent predictors for hemorrhagic stroke. Neither tHcy, nor the two MTHFR polymorphisms were significant predictors for ischemic strokes. The second was a prospective long-term follow-up study within the VIP and MONICA cohorts to determine whether a first-ever myocardial infarction (AMI) causes increased levels of tHcy. Fifty cases developing AMI after the first screening participated in a second screening (mean follow-up 8.5 years) with 56 matched referents. Increase in tHcy did not differ between cases and referents. tHcy was related to AMI at follow-up, but not at baseline and no longer significant after adjusting for creatinine and albumin. The third was a method study to determine if cystatin C, creatinine, albumin and other lipoprotein risk markers of cardiovascular disease could be analysed in Stabilyte™ plasma stored at -80°C. It was found to be suitable for all analyses tested and using this tube would simplify sampling for epidemiological studies. The fourth study was a prospective longitudinal long-term study of 735 subjects (340 men and 395 women, age 25-64 at first screening), participating in two MONICA screenings nine years apart, who donated blood in Stabilyte™ tubes to study change over time in tHcy and its determinants. We confirmed the age dependency in a cross sectional setting. In contrast, if followed longitudinally over time, no change in tHcy or in the prevalence of hyperhomocysteinemia was found. Cystatin C and creatinine increased, and albumin decreased. In multivariate models baseline levels of albumin, creatinine, cystatin C, and to some extent hs-CRP, were predictors of tHcy at follow-up but gender differences were seen. Age was not a major determinant of change in tHcy over nine years. In conclusion, tHcy and MTHFR are risk factors for first-ever haemorrhagic, but not ischemic stroke in a prospective setting. A first myocardial infarction does not cause an increase in tHcy. No long-term changes were seen in tHcy over a nine-year period in neither men, nor in women.