BACKGROUND: Assertive Community Treatment (ACT) teams have become a part of mental health services for people with severe mental illness in many high-income countries. Studies in several countries have investigated the outcomes of ACT, and knowledge is also needed about outcomes of ACT teams in Norway. Our aims were to study clinical outcomes of ACT, how the outcomes were associated with characteristics of patients and treatment, and whether they differed across ACT teams. METHODS: Our explorative, prospective, pre-post multicenter study involved 142 patients who received ACT for two years from the first 12 ACT teams established in urban and rural areas of Norway. There was no control group. The primary outcome was change in clinician-rated psychiatric symptoms. Secondary outcomes were clinician-rated change in functioning and engagement and change in community tenure compared with 2 years prior to ACT. We measured fidelity to the ACT model using the Tool for Measurement of Assertive Community Treatment. We performed linear mixed-effects modeling to analyze outcomes and their associations with characteristics of patients and treatment. RESULTS: After two years, psychiatric symptoms were significantly reduced with a small effect size. Negative symptoms, anxiety and depression, and agitation and mania had significant reductions, while positive symptoms had nonsignificant changes. Functioning, engagement, and community tenure all significantly increased with small effect sizes. Age, difficulty to engage, problematic use of alcohol, frequent previous use of inpatient services, total number of sessions, and team's fidelity to the ACT model were associated with different groups of symptoms. Less improvement in functioning was associated with team fidelity and number of sessions. Change in engagement was not associated with any predictors. Increased community tenure was greater for younger patients and patients who were on community treatment orders at treatment start. CONCLUSIONS: ACT for two years led to significant positive outcomes with small effect sizes for psychiatric symptoms, functioning, engagement, and community tenure. The outcomes were associated with some potential predictors, and some team-level variance emerged. Positive significant outcomes after two years indicate that larger improvements may be achieved from longer-term treatments by ACT teams.