In this paper the authors present an evaluation of the effectiveness of suspended prison sentences, as measured by the rate of reversion to crime in the period of probation. The study was designed so that the history of individual offenders could be traced within a large and representative research sample. The analysis begins with a presentation of quantitative data of the sample’s criminal history including that related to their suspended prison sentence. This is followed by conclusions and a summary of the empirical data. On the basis of the covariance analysis the authors conclude that suspended prison sentences have a moderate impact on the prevention of further offences by the criminal. The study was conducted by the Institute of Justice in Warsaw in 2012 using a data sample of 1894 criminal convictions. It is worth mentioning that within this study there was a maximum follow-up period of five years after the imposition of a suspended sentence. The survey also obtained initial data on the convicted criminals given the suspended sentence so it was possible to observe and assess their criminal activity even over several years. For this reason, the research may claim to be longitudinal.