High-cost patients are a small group of health care users who account for a relatively large proportion of resources. Among these patients, our literature review confirmed that limited work has focused on those who use a high amount of mental health and addiction services. We found that the majority of these patients’ costs were due to psychiatric hospitalizations, some of which could be reduced through better care management. Among this mental health high-cost subgroup, our predictive model identified those at risk of any psychiatric hospitalization in Ontario. Identified patients were more likely to be male, younger, and a first-generation immigrant; live in low-income neighbourhoods or rural areas; have a past hospitalization with a main diagnosis of mood disorder; and have a high number of recent psychiatric hospitalizations. For some of these identified patients, our business case analysis supported investments in interventions that could reduce unnecessary hospitalization costs.