We examined the relationship in a large-living population between the concentration of triglycerides in plasma from fasting persons and the concentration of very-low-density lipoprotein cholesterol (VLDL-C) as determined after preparative ultracentrifugation. Data from individuals in the Framingham Heart Study who were free of coronary heart disease, did not have Fredrickson Type I, III, or V hyperlipoproteinemia, and whose plasma-triglyceride values were less than 4000 mg/L were considered in the analyses. A total of 4100 Framingham Offspring, ages 20-49 years, met these criteria, as did 2284 participants in the original Framingham Cohort, aged 50-79 years. We estimated a linear regression model with an intercept and slope coefficient, using VLDL-C (units, mg/L) as the dependent variable and triglycerides as the dependent variable. The intercept was approximately 30.0 mg/L, and the slope coefficient for triglycerides was about 0.20 when all age and sex groups were combined. However, when the analysis was performed by age group, the slope coefficient for triglycerides increased with each age group only for women. The model, in which a different intercept and slope is used for each age and sex group was statistically better than a previously developed simpler model. On the other hand, the older model, in which VLDL-C is easily estimated by multiplying the triglyceride value by 0.20, is reasonably accurate and is very simple to use. The generalizability of the model is discussed in the light of results from other population studies.