Prevalence data on mental retardation is presented including international estimates on general prevalence, age directions, geographical variations within the United States, racial and ethnic variations, economic class distributions, family variations, and population distribution in institutions. Statistics are also provided in areas of specific diseases and conditions associated with mental retardation such as prenatal and postnatal infections (both known and unknown), intoxications, trauma or physical agents, and metabolism or growth effects. Seventy seven tables and twenty one illustrations accompany the epidemiological information. (RD)