Information on the 100 largest U.S. jails was obtained from Bureau of Justice Statistics data. These 100 jails served approximately 67 cities or towns. The analysis examined the relationship between the percentage change in the jail inmate population between 1978 and 1982 and changes in index crimes per 100,000 population (excluding arson) between 1979 and 1983 for 59 of the original 67 cities. As in previous research, the study found little relationship between changes in jail population and the total index crime rate. In contrast, analysis did provide tentative evidence that increasing jail population has a moderate negative impact on property crimes, especially burglary and larceny, and a weaker impact on aggravated assault as a violent crime. An unexpected finding is that all correlations for each separate index crime are negative, though not significant for murder, rape, robbery, and motor vehicle theft. Directions for future research and implications of the study's results for correctional policies are discussed. Tables and over 20 references.