We sought to estimate the number of arrestees at risk for inadequately treated drug and alcohol withdrawal in US jails. We used Arrestee Drug Abuse Monitoring Program data to estimate prevalence rates of alcohol and opiate dependence. Our results revealed rates of alcohol and opiate dependency among arrestees of approximately 12% and 4%, respectively; only 28% of jail administrators reported that their institutions had ever detoxified arrestees. Inadequately treated drug and alcohol withdrawal in US jails appears widespread. Our data raise important ethical and constitutional questions.