Little research has been conducted to date exploring the extent to which treatment can impact upon dynamic factors thought to be related to sexually harmful behaviour in adolescents. This study explores the within-treatment change in a group of adolescent males who have sexually harmed. Pre- and post-treatment psychometric data for 34 participants who had completed an offence-specific group work programme while residing at a specialist residential treatment provision for adolescent males who have sexually harmed were analysed to investigate if the treatment had affected their scores on measures of psychosocial functioning and offence-related attitudes. Results supported the hypothesis that treatment would impact positively on the post-treatment psychometric scores, although the limitations of the study hinder the extent to which any firm conclusions can be drawn. Some corroborating data from relevant domains on the ERASOR risk assessment protocol is also presented for a subgroup of 24 individuals. (Contains 8 tables.)