Background: Mean concentrations of total cholesterol (TC) among adults have declined in the United States for decades. Whether the decline has been owing to prevention of high TC levels or treatment of high TC levels once present is not known. Objective: To determine whether population-wide influences and/or the high-risk approach have been operating to produce the well-known decline in mean TC concentration in the US population. Methods: We examined changes in the distribution of TC levels across US birth cohorts as sampled in the National Health Examination Survey and the National Health and Nutrition Examination Surveys I, II, and III. We tested the hypotheses that the age-adjusted 10th, 25th, 50th, 75th, and 90th percentiles of TC levels were lower in more recent US birth cohorts than in earlier cohorts. Results: Data were analyzed for 49536 participants born between 1887 and 1975 and examined at ages 18 through 74 years between 1959 and 1994. The 10th, 25th, 50th, 75th, and 90th percentiles of TC levels (adjusted for age, race, and sex) were estimated to be lower by 3.4, 3.9, 4.7, 5.7, and 7.1 mg/dL (0.09, 0.10, 0.12, 0.15, and 0.18 mmol/L), respectively, for every successive 10 years in date of birth (P<.001 for each estimate). Conclusions: The declines in TC levels associated with successive birth cohorts were greater at the upper aspect of the distribution, probably because of the combination of population influences and treatment effects. The differences seen at the lower percentiles support the contention that a strong prevention effect occurred in the US population from 1959 through 1994. Greater understanding of this dramatic shift in the distribution of TC levels could support future prevention efforts.