In this article, the effectiveness of a psychological treatment program for men who are in prison for committing a serious offence of gender violence is tested. The sample consisted of 52 men who were imprisoned in eight Spanish prisons. The psychological treatment was a cognitive-behavioral program, in a group format, with 20 weekly sessions that lasted for 8 months. The results showed a significant improvement of irrational beliefs, about both women and violence, as a strategy to cope with everyday difficulties and a significant decrease of psychopathological symptomatology, anger, and hostility. On the other hand, results indicated that the only difference between the patients who dropped out of treatment and those who completed it was the level of initial motivation for treatment. Implications of this study for clinical practice and future research in this field are discussed.