We estimate the impact of determinate sentencing laws (DSLs) on prison commitments, prison populations, and Uniform Crime Report crime rates. Ten states enacted these laws between 1976 and 1984; all abolished parole and most established presumptive sentences. The research uses a multiple time‐series design that, among other benefits, controls for national trends and facilitates the use of control variables. We found that DSLs are clearly associated with prison population growth in only one state, Indiana, and with major reductions in two, Minnesota and Washington. The remaining laws show no evidence of increasing populations and may have reduced them somewhat. The estimated impacts on commitments are similarly varied. There is little or no evidence that DSLs affect crime. Earlier studies evaluating individual DSLs are often criticized for poor research designs, and our findings support the criticism