Two types of adult offenders were empirically identified by a clustering procedure using eleven personality measures. Although neither group treatment nor offender-type had a significant main effect on three measures of recidivism at one year following release, there was evidence of a treatment by offender-type interaction on two of the recidivism measures. Offenders who were classified high in self-image showed greater improvement in the group therapy program (i.e., fewer reconvictions, and lower reconvicted offense severity), while offenders who were low in self-image did more poorly in group therapy than under institutional care. The results are discussed in terms of the need to develop differential treatment models in the design of correctional programming for adult offenders. (Publisher abstract)