Although prison-based therapeutic communities have conceptualised their treatment model as one incorporating social learning and interpersonal approaches, little research has been conducted to evaluate how far they are effective in modifying maladaptive interpersonal styles. This study aimed to evaluate the efficacy in promoting adaptive styles of interpersonal relating. Changes in relating style were measured over an 18-month period during treatment at Grendon. The third version of the Person's Relating to Others Questionnaire (PROQ3; Birtchnell et al., submitted), a measure of interpersonal relating styles, was administered to 130 prisoners on arrival at Grendon, after 9 months and after 18 months. Significant improvements in relating styles were demonstrated after 9 months and improvement was maintained between the 9 and 18-month intervals. Twenty-two per cent of participants showed statistically reliable improvement in terms of their PROQ3 total score after 18 months. It is concluded that the therapeutic community treatment model at HMP Grendon is effective in producing positive changes in interpersonal relating in personality disordered offenders. (PsycINFO Database Record (c) 2019 APA, all rights reserved)