Mean levels of total cholesterol by sex and age were compared between groups of whites in visit 2 of the Lipid Research Clinics Program Prevalence Study and in the second National Health and Nutrition Examination Survey (NHANES II). NHANES II estimates were found to be higher than comparable visit 2 estimates by 3-14 mg/dl for males and by 11-21 mg/dl for females. Adjustment for the use of differing components of blood in making the lipid determinations in the two studies (plasma in visit 2 and serum in NHANES II) reduced the original differences by roughly one half. Other adjustments--for operational differences among laboratories that performed the blood sample analysis in the two studies, for educational differences in the study populations, and for inclusion of data from a Toronto clinic in the visit 2 data--were collectively found to explain most of the remaining differences between visit 2 and NHANES II, especially for males.