Background The purpose of this study was to examine demographic, background, and psychopathology variables that predict length of stay and readmission among youth with mental health needs. Method We analyzed data on 2445 youth who were admitted into adult psychiatric beds in Ontario, Canada. Multiple regression was used to examine length of stay, whereas logistic regression was used to examine the predictors of readmission. Results Youth were likely to stay longer in hospital if they were older, were boys, had a diagnosis of schizophrenia, mood disorders, eating disorders, personality disorders, and intellectual disability. Education, discharged against medical advice, and a diagnosis of adjustment disorders were all associated with shorter length of stay. Age, living in a group home or assisted care, a diagnosis of schizophrenia, mood disorders, and intellectual disability predicted readmission. Conclusion Strategies to improve current psychiatric services (e.g. how to reduce psychiatric hospital readmissions) are discussed.