Enumerations of the mentally ill and mentally retarded were included in the six U.S. censuses conducted between 1840 and 1890. Inclusion of these categories reflected the new concern for the mentally ill and mentally retarded that was emerging at that time as part of a new social consciousness. Dr. Gorwitz analyzes these census results, considers the limitations of the data, and discusses the factors that led to discontinuation of the enumerations of the two groups after the 1890 census.