Expectations of increased prison populations were associated with the enactment of the 1978 Illinois determinate sentencing law (DSL). These expectations were based in the numerous probation-disqualification provisions for class x felonies, the encouragement for judges to impose relatively short prison terms in marginal cases, and the legislators' intention that the DSL would satisfy the public's desire for tougher sentencing. A review of Illinois incarceration rates for 1961-82 shows a steady and dramatic increase in rates after a decline from 1969 through 1973. Parallel incarceration patterns in States without determinate sentencing laws, however, suggests that DSL's may only be one factor contributing to prison overcrowding. The data on prison populations and parole before and after the enactment of the DSL indicates that prior to 1978, parole was used as a safety valve to control prison populations during periods of increased admissions. The DSL removed this safety valve, thus contributing to prison overcrowding. Alternatives for addressing prison overcrowding include an increase in prison capacity, discretionary release, and shorter sentences. 40 footnotes, 1 table, and 2 figures.