Most Spanish prisons provide specialised treatment for incarcerated sex offenders, both rapists and child molesters. This treatment is a cognitive-behavioural intervention that has shown relative effectiveness in previous research. With regard to offenders' rehabilitation, recidivism assessments are necessary as a final measure of treatment effectiveness. However, the evaluation of recidivism by itself does not provide sufficient information on the treatment process and the specific effects that treated subjects could undergo. This paper aims to analyse the therapeutic effectiveness of psychological treatment provided to rapists (in general, males sentenced for committing a sexual offence against women). To this aim, a group of treated rapists (N = 153) serving a sentence in prison was analysed. Using a specially designed scale (PASSO), the global therapeutic change and ten specific variables (including assertiveness, readiness to change, cognitive distortions, impulsivity, etc.) were assessed. The within-subjects comparison showed that treated sex offenders improved, in therapeutic terms, globally as well as in most of the specific variables assessed (improvements not experimented by the control group). Also, different therapeutic subscales showed relevant associations between them. The findings regarding treatment effectiveness are discussed.