This paper describes new midyear (July 1) estimates of the “true” population of the United States by age, sex, and color (white, nonwhite) for the 1940s and 1950s. It also presents the corresponding implied coverage estimates for the 1940 and 1950 censuses. The new population estimates are calculated by combining the most recent figures on the 1960 population with estimates of the demographic components of change for the 1950s and 1940s in an iterative reverse cohort-component projection algorithm. Among the principal findings of the new estimates are: (a) existing midyear estimates of the “true” population in the 1950s are 450,000 to 500,000 too high; (b) existing age-specific estimates for the 1950s tend to underestimate the population at the older ages (55 years and over) and overestimate the population in the young and middle adult years (15 to 54 years); (c) estimates of the “true” population in the 1940s were too low except for nonwhites at ages 65 and over; (d) existing estimates of percentage net undercount and underenumeration for the 1950 and 1940 censuses tend to be too high, substantially so for nonwhites in the 1940 Census; and (e) nonwhites were more completely enumerated in 1940 than in 1950. Thus, in addition to being methodologically and temporally consistent with post-1960 estimates, the new population estimates described here imply some substantial revisions in demographic, social, and economic statistics for the two decades prior to 1960.