The follow-up exam in 1986-87 collected baseline data on cognitive performance in 622 individuals, including 132 monozygotic and 134 dizygotic twin pairs. Beginning in December, 1994, a research grant was used to conduct a fourth exam in the surviving subjects of this cohort that used this rich resource of available data to determine the relationship between cardiovascular risk factors collected over 23 years of follow-up, changes in cognitive function since the last exam, and brain morphology assessed by magnetic resonance imaging (MRI). Given the mean age of this cohort (72 years) in 1995, the next several years represented a critical time to monitor age related changes in brain morphology and cognition. The addition of a fourth exam made the NHLBI Twin Study the longest longitudinal study of cardiovascular disease epidemiology in twin subjects (23 to 25 years of follow-up) and the first to investigate the contribution of genes and the environment to associations between CVD risk factors, cognitive performance, and brain morphology. There were five specific objectives of this study. The first was to evaluate the prospective and cross-sectional relationships of cardiovascular risk factors collected over 23 years of follow-up (e.g., blood pressure, lipids, obesity, smoking) and indices of brain morphology assessed at the fourth exam. The second objective was to determine the extent that twins discordant for essential hypertension or non-insulin-dependent diabetes mellitus (NIDDM) or cardiovascular disease (CHD) showed different MRI profiles. The third was to characterize the neuropsychological changes in performance from Exam 3 to Exam 4 and determine the contribution of genes and the environment to stability or change. The fourth objective was to use the data from exams 1 to 3 to investigate prospective relationships between CVD risk factors and decline in cognitive performance from Exam 3 to Exam 4. The fifth was to determine the heritability of the new measurements obtained for the first time at the fourth examination cycle (e.g., MRI measures of infarct area and localization, white matter hyperintensities (WMHI), and ventricular volumes).