Total plasma lipid and lipoprotein-cholesterol distributions of 4756 white men and women ages 20-59 years are presented. Measurements were obtained during the visit-2 survey of the Lipid Research Clinics Program Prevalence Study and correspond to a 15% random sample of 35,748 white adults screened during the LRC visit-1 survey. Standardized examinations were carried out by 10 North American clinics using a common protocol, on diverse target populations chosen to include a range of sociodemographic characteristics. Age-specific means, medians and selected percentiles are given by sex, with stratification on exogenous sex hormone use in women. Plasma lipid and lipoprotein concentrations in men and women vary with age. Differences in lipid and lipoprotein levels between the study populations are also present and manifest themselves as parallel trends of age-related changes in the 10 populations examined. Higher total cholesterol values in men compared with women appear between the ages 20-50 years and higher LDL cholesterol between the ages 20-55 years. VLDL cholesterol levels are similar in both sexes at ages 20 and 59 years, but higher in men than in women in all intermediate age groups. HDL cholesterol is higher in women than in men throughout the range considered. Women taking sex hormone preparations have higher total cholesterol than women not on hormnes between ages 20 - 50 years, and higher LDL cholesterol between ages 20-40 years. From the third age decade onward, HDL cholesterol levels are progressively higher in women taking hormones than in women not taking sex hormones. Compared with women not taking exogenous sex hormones, women taking hormones have higher total plasma triglyceride values at all ages from 20-59 years. VLDL cholesterol values are higher in women on hormones compared with nonusers of hormones younger than 55 years.