This article argues that the conclusions of previous research on the impact of sentencing guidelines may be misleading due to the cross-sectional methodologies employed in these studies. This study will suggest that a theoretically driven longitudinal analysis of mandatory guidelines offers a more appropriate way to study these policies. Specifically, the author proposes that over time, guidelines that link sentencing decisions to correctional resources help to mitigate prison populations, while those that do not tend to contribute to growth in that area. An analysis of prison populations in the American states between 1975 and 1998 confirms this proposition. The results suggest that mandatory guidelines have increased both commitment and incarceration rates in states where sentencing decisions are not resource driven. Alternatively, when mandatory guidelines are linked to capacity and expenditures, these policies have had either a negative or an insignificant impact on prison populations. (Contains 2 tables and 11 notes.)