Previous studies have shown that stressful life events (SLEs), gender, social functioning and pretreatment severity are some of the predictors and/or moderators of treatment outcome in psychiatric care. The current study explored the effect of these predictors and moderators on the treatment outcome related to assertive community treatment (ACT) proposed to young people with severe mental disorders. 98 patients were assessed for externalizing and emotional difficulties, at admission and then at discharge of an ACT. Analyses revealed significant improvements in terms of symptomatology. In particular, regression analyses showed that pretreatment severity is a significant predictor of the outcome on emotional symptoms and is moderated by SLE on the outcome on externalizing symptoms. Furthermore, higher social functioning proved to predict better outcome on externalizing symptoms. Our results further evidence that these factors can explain inter-individual differences in outcome related to ACT. The theoretical and clinical implications of these results are discussed.