Cross-sectional age- and sex-specific plasma lipid distributions (means, medians and selected percentiles) are given for 48,431 white participants in visit 1 of the Lipid Research Clinics (LRC) Prevalence Study. This study consisted of two visits in which 10 LRCs screened participants selected from well-defined North American target populations that included a broad range of sociodemographic subgroups. These data confirm findings from earlier studies in developed countries, showing age-related differences in plasma lipid levels. However, for overall distributions, the LRC data showed slightly lower cholesterol and markedly higher triglyceride values than those previously reported for North America. Some variation in plasma lipid values was evident among the clinic populations. The large number of participants within most subgroups permitted a variety of analytic and comparative studies. For example, data from the large pediatric population revealed a drop in plasma cholesterol levels in adolescent males and females. Males aged 20--50 years had higher cholesterol levels than females in the same age group, and higher triglyceride levels between ages 20--70 years. Numbers were also sufficient for meaningful comparisons between lipid distributions of females who were taking sex hormones and those who were not: In females taking sex hormones, cholesterol and triglyceride levels were higher for subjects younger than 45 years, but slightly lower after age 45, than lipid levels in females not taking hormones.