A recent study by Abt Associates (Abt/Carlson) purported to show that increments to prison capacity would lead to growth in prison population to fill that added capacity two years later. That finding has rapidly attained broad circulation and widespread acceptance. The original conclusion was based on a coefficient of 1.02 in a simple regression equation that represents change in prison population as a function of lagged changes in prison population and capacity. Reanalysis of the data shows that the original estimates resulted from a computation error; when that error is corrected the coefficient estimate is reduced to .264. Furthermore, two data points were particularly influential in the regression analysis, and omitting them results in a coefficient of .095 which is not statistically significant. Thus, the coefficient on which the original conclusion was based is eliminated in importance. It was unreasonable to expect valid results from so simplistic a model with no consideration of other exogenous factors (e.g., state budgets, court orders, public attitudes, degree of prison overcrowding) and without consideration of the simultaneous effects of projected prison population in influencing the creation of prison capacity. This paper presents the reanalysis of the Abt data, and conducts more detailed analyses of the trends in individual states. During the late 1950s and 1960s, there was considerable spare capacity in state prisons, with no clear effect of that spare capacity on prison population. (Author abstract)