Estimates of the civilian noninstitutional population define the sample frame of most major national sample surveys and are used as denominators in the computation of prevalence rates for various sociodemographic phenomena, such as labor force participation and school enrollment. Current U.S. Bureau of the Census methods for making intercensal estimates and postcensal projections of the civilian noninstitutional population rest on the assumption that the age-, sex-, and race-specific proportions of the population that are institutionalized—as estimated by the last census—remain constant until the next census. This article examines the empirical validity of this assumption by using data from the decennial censuses for 1940–1980 and, in light of substantial decade to decade changes in the age patterns of the institutional proportions for sex- and race-specific populations, seeks to develop alternative methods. To pursue the latter objective, parametric curves are fit to the age-specific institutional proportions for each population for each decade. A study of the observed historical variation in the parameters of these curves then leads to some suggestions about how their shapes can be estimated between censuses and projected beyond the latest available census to provide more accurate estimates and projections of the civilian noninstitutional population.