The population living in institutions is excluded from most major national surveys. We evaluate the implications of this exclusion for disability statistics and research by compiling and examining existing disability information for the population living in institutions, with an emphasis on working-age people. The population living in institutions is a very small share of the entire population, but increased considerably from 1990 to 2000, especially for those ages 18-64. Working-age people accounted for a much larger proportion of the population living in institutions in 2000 (56 percent) than in 1990 (45 percent). As of 2000, 86 percent of the institutionalized working-age population resided in correctional institutions, and the remaining 14 percent were approximately evenly split between nursing homes and other institutions, many of which specialize in care for people with disabilities. When disability is defined as having at least one of the four disabilities in the 2000 Census—self-care, mental, physical, or sensory disabilities—the vast majority of the population with disabilities lives in household units; 8.7 percent live in group quarters (GQs), 6.4 percent live in institutional GQs, and 2.3 percent in non-institutional GQs. For working-age people with disabilities, the share of males living in institutional GQs is much larger than the share of females (7.7 percent versus 1.7 percent), in part reflecting the fact that more than nine out of ten inmates in correctional institutions are male. Working-age people with disabilities residing in institutional GQs are also disproportionately African American (38.6 percent of those ages 18-49 and 22.4 percent of those ages 50-64). Increased incarceration rates and the relatively high prevalence of disability in the incarcerated population suggest that growth in incarceration could have a substantial impact on disability prevalence in the household population, and on the characteristics of the household population with disabilities, most notably for young male African Americans. The nursing home residence rate declined for all age groups, but for those under 65 the decline is very small relative to the size of all persons in that age group, and thus seems unlikely to have much impact on disability statistics for the household population. We found no surveys covering the population living in institutions other than correctional institutions and nursing homes. The lack of information on this population may present a substantial problem for disability statistics and research.