In 1994 and 1995, the National Health Interview Survey included a Disability Supplement (NHIS-D) to collect extensive information about disabilities among individuals sampled as part of annual census-based household interview surveys. Here we describe the development and application of operational definitions of mental retardation and developmental disabilities to items in the NHIS-D to estimate prevalence. In our analyses, we estimate the prevalence of mental retardation in the noninstitutionalized population of the United States to be 7.8 people per thousand (.78%); of developmental disabilities, 11.3 people per thousand (1.13%); and the combined prevalence of mental retardation and/or developmental disabilities to be 14.9 per thousand (1.49%). Differences in prevalence estimates for mental retardation and developmental disabilities and among people of various ages are explored.