Context: Approximately 1.8 per 1000 Ontario children are placed in out of home care due to child maltreatment. Children living in out of home care have complex mental, behavioural, and psychosocial needs and have a significantly higher prevalence of emotional and behavioural problems compared to the general population. Between 20-40% of U.S. children living in foster care are prescribed three or more classes of psychotropic medications. Little is known about psychotropic polypharmacy among Canadian children living in out of home care. Research evidence for efficacy and safety of using multiple psychotropic medications in children is lacking. Objective: To determine the prevalence and determinants of psychotropic polypharmacy among children in care in the Kingston, Ontario region. Study Design: Retrospective cohort study. Dataset: Family and Children’s Services of Frontenac, Lennox and Addington (FACSFLA) database includes demographic (child date of birth, sex, ethnicity) and child welfare data (duration in care, and duration of and reason for child welfare involvement). Third party insurance data includes medications, prescription dates and quantities dispensed from Jan. 1, 2012 to Dec. 31, 2017. Population studied: A study cohort of children living under FACSFLA care during the study period. Outcome measures: Prevalence of psychotropic medication and of polypharmacy (three or more psychotropic agents taken concurrently). Predictor: Total duration in care. Covariates: Child age, sex, ethnicity, diagnosed mental health conditions, comorbidity of mental health conditions, reason for child welfare placement. Results: Characteristics of the children and the prevalence of use of psychotropic medications will be described using descriptive statistics. The relationship between duration in care and polypharmacy will be modeled using generalized estimating equations. Conclusions: This project would address an important gap in understanding psychotropic prescribing to children in care in Ontario. Children’s services in Ontario are frequently poorly integrated. Children in care are especially vulnerable to receiving fragmented care, as they are subject to frequent transitions (between homes, child welfare staff and health care providers) resulting in potential gaps in information continuity and advocacy. Results from this study will be important for improvement of mental health care for a vulnerable group of children and youth.