In response to Congressional mandate and under the aegis of the Federal Agency for Toxic Substances and Disease Registry (ATSDR), a comprehensive report to Congress on childhood lead poisoning in the United States was prepared. We have examined numbers of lead-exposed U.S. children by socioeconomic/demographic strata for children 0.5 to 5 years of age; by children in U.S. lead-screening programs; and by enumerations of children 0.5 to 5 years old in the oldest (i.e., highest paint lead and lead plumbing) housing. Using blood lead (PbB) prevalence projection modeling and data of the Second National Health and Nutrition Examination Surgery (NHANES II), it is estimated for 1984 that 2.4 million black and white children 0.5 to 5 years old in metropolitan U.S. had PbB levels greater than 15 micrograms/dL. For all races and the entire nation, we estimate 3 to 4 million children will have PbB levels greater than 15 micrograms/dL. Inner-city, low-income children have the highest prevalences of PbB levels above this criterion level, but sizable numbers of all strata of children have elevated PbB levels when considering both base populations and prevalences for the specific strata (total of 30 strata). Lead screening programs indicate much lower numbers of exposed children compared to NHANES II-based projections, for various reasons that allow programs to underestimate true prevalences. Analysis of 1980 U.S. Census Bureau housing data for 318 standard metropolitan statistical areas show that 4.4 million children 0.5 to 5 years old live in the oldest U.S. housing (pre-1950). Of these, most are actually in the more affluent socioeconomic strata.