Objective: Statistical inferences based on routine outcome monitoring data are susceptible to biases. Because this process may be influenced by differences in attrition and treatment duration, we wished to gain an insight into the relationship between treatment duration and clinical outcome. Method: We enrolled 569 assertive community treatment (ACT) team patients. As part of a six-monthly routine outcome monitoring (ROM) procedure, we used the Global Assessment of Functioning (GAF) scale, the Health of the Nation Outcome Scales (HoNOS), and a scale to assess their treatment motivation and satisfaction with services. Duration of ACT showed that treatment duration was short for 292 patients [<= three ROM assessments; 11.6 months (SD = 6.1)], medium for 191 [four to six ROM assessment Results: More patients receiving long-term ACT were diagnosed with a psychotic disorder and/or substance abuse than those whose treatment was shorter. Patients whose treatment lasted longer had worse baseline GAF and HoNOS scores than those whose treatment was shorter. Structural equation modelling showed that the interdependencies between determinants and outcome variables (concerning the relationships between both identical and non-identical variables over time) were different for each of the Conclusions: Patients in ACT teams with different treatment durations constitute distinguishable groups with different outcomes. This should be taken into account when using outcome data for benchmarking purposes.