This simulation study investigates the effects of within-individual variability in estimated cardiovascular risk on categorization of patients as high risk. Published estimates of within-individual blood pressure and cholesterol variability were used to generate blood pressure and cholesterol levels for hypothetical subjects at a range of ages. These were used to calculate the estimated cardiovascular risk of each individual. The relationship between an individual's mean cardiovascular risk and within-individual coefficient of variation for cardiovascular risk was determined. Using the derived relationship, mean cardiovascular risk and within-individual variation in risk was calculated for 5018 adults from a population health survey. From this, was determined their probability of being classified as high risk (>20% 10-year cardiovascular risk) and the test characteristics of risk estimation at a range of ages. Within-individual variability in cardiovascular risk and potential for misclassification are both greater in lower-risk populations. At age 35-44 years, the positive predictive value of a diagnosis of high risk is 0.61 (95% confidence interval (CI): 0.59-0.64), and at age 65-74 years, it is 0.94 (95% CI: 0.91-0.96). About 39% of adults under 45 years diagnosed as high risk are not at high risk. Cardiovascular risk assessment should be targeted at high-risk populations.