"In 2007, the criminal justice system was the largest single source of referrals to the substance abuse treatment system, comprising 37 percent of all admissions in the Treatment Episode Data Set (TEDS) (approximately 670,500 of the 1.8 million admissions). Moreover, the majority of these referrals were from parole and probation offices (44 percent of criminal justice admissions where detailed criminal justice source information is known)."
Source
Substance Abuse and Mental Health Services Administration, Office of Applied Studies. (August 13, 2009). The TEDS Report: Substance Abuse Treatment Admissions Referred by the Criminal Justice System. Rockville, MD.