This study measured the prevalence of current, severe symptoms of a mental health problem in an adult population of inmates in Ontario, Canada. The Resident Assessment Instrument–Mental Health was used to measure the prevalence of symptoms among a sample of 522 inmates. Propensity score weighting was used to adjust for nonrandom selection into the sample. Prevalence estimates were derived for the total inmate population, remand and sentenced, males and females, and Aboriginal and non-Aboriginal inmates. It is estimated that 41.1% of Ontario inmates will have at least one current, severe symptom of a mental health problem; of this group, 13.0%, will evidence two or more symptoms. The number of symptoms is strongly associated with presence of a psychiatric diagnosis and level of mental health care needs. Female (35.1%) and Aboriginal (18.7%) inmates are more likely to demonstrate two or more current, severe symptoms. Greater efforts must be made to bridge the gap between correctional and mental health care systems to ensure inmates in correctional facilities can access and receive appropriate mental health care services.