RESUMEN
Millions are confined in U.S. jails each year, often with unmet health and social needs. After release, many will visit the emergency department (ED). To illuminate their patterns of ED use, this study linked records from all individuals detained at a Southern urban jail over a 5-year period with health records from a large health care system with three EDs. Over half used the ED at least once, and of those who received care at the health system, 83% visited the ED. Jail-involved people made up 4.1% of the health care system's ED users but 21.3% of its chronic frequent ED users. Frequent ED use was associated with more frequent jail bookings and with co-occurring serious mental illness and substance use disorder. Health systems and jails have a common interest in addressing the needs of this population. Individuals with co-occurring disorders should be prioritized for intervention.