Although a large number of offenders are released to parole each year, little is known about the effectiveness of parole supervision in reducing re-offending. The few studies that have been conducted provide mixed results and, for the most part, have been unable to rule out the possibility of selection bias. The present study is the first to evaluate the effectiveness of parole supervision using propensity score matching techniques. It compares two groups of offenders, carefully matched in terms of factors likely to affect re-offending but differing in terms of whether they are supervised. The results suggest that parole supervision does reduce the risk of re-offending.