Aims.To study the systematic assessment of need for care and clinical parameters for use in treatment plans in patients diagnosed with severe mental illness.Methods.The Cumulative Needs for Care Monitor (CNCM) includes various validated instruments, such as the Camberwell Assessment of Need. A Markov-type cost-effectiveness model (health care perspective, 5-year time horizon) was used to compare CNCM with care as usual (CAU). Two studies were used to determine model parameters: a before–after study (n = 2155) and a matched-control study (n = 937).Results.The CNCM may lead to a gain in psychiatric functioning according to the models. CNCM patients remain in (outpatient) care, while CAU patients drop out more frequently. There is only a small difference in inpatient care. As a result, average costs per patient in the CNCM group are between €2809 (before–after model) and €5251 (matched-control model) higher. The iCER was between €45 127 and €57 839 per life year without psychiatric dysfunction gained.Conclusions.CNCM may be only cost-effective when willingness to pay for a life year without psychiatric dysfunction is higher than €45 000. However, this result is highly sensitive to the level of psychiatric dysfunctioning in patients who do not receive care.