This study tested evidence for antisocial attitudes as a mechanism of change in offender treatment by examining whether the Measures of Antisocial Attitudes and Associates (MCAA) and within-treatment change in scores on this scale have predictive validity for risk of reoffending. Pretreatment and posttreatment scores on the MCAA were obtained from a large sample of 1,858 offenders who had completed offender treatment programs while in custody (n = 854) or in the community (n = 1,004). Individual within-treatment change was calculated with simple difference scores as well as categorizations of clinically significant change. Results showed that discrete scores on a number of MCAA measures were associated with reoffending hazard at pretreatment and at posttreatment. Change over treatment was also highly significant at the group level whereas a modest proportion of offenders achieved clinically significant change. None of the measures of within-treatment change were significantly associated with reoffending, however. The results suggest that self-reported antisocial attitudes may not be a valid indicator of causal mechanisms of treatment or change in offenders’ risk of reoffending as a result of completing treatment.