Objective: The implementation of assertive community treatment (ACT) varies widely. To date, the association between model fidelity and effect has not been investigated in Europe. We investigated the association between model fidelity and outcome in the Dutch mental health system. Method: In a prospective longitudinal study, ACT model fidelity and patient outcomes were assessed in 20 outpatient treatment teams. Patients with severe mental illness ( n = 530) participated in the study. Outcomes were assessed 3 times using the Health of the Nation Outcome Scales (HoNOS), the Camberwell Assessment of Need Short Assessment Schedule (CANSAS), and the number of hospital days and homeless days during a 2-year follow-up period. Data were analyzed using multilevel statistics. Results: High ACT model fidelity was associated with better outcomes on the HoNOS and less homeless days. Among all of the ACT ingredients, team structure was associated with better outcomes. No associations were found between ACT model fidelity, number of hospital days, and CANSAS scores. Conclusions: Our evidence supports the importance of model fidelity for improving patient outcomes.