This article presents the basic methods and populations of the Lipid Research Clinics (LRC) Program Prevalence Study. The purpose is to provide a frame of reference for the other articles in this monograph; hence, the populations and major data collected are described briefly, as are procedures for enhancement of data quality and approaches to statistical analysis. More detailed descriptions for some procedures are included in the pertinent articles. The two sequential screening examinations of the Prevalence Study provided 60,502 participants from centers in the United States and Canada for the first phase and 13,852 for the second. The monograph is based on data from 11,657 white and 644 black participants aged 6 years or older screened at both visits.