Objective: The authors sought information on the number, structure, and effectiveness of programs aimedat diverting mentally ill inmates from the criminal justice system into the mental health treatment system. Methods: A working definition ofajail diversion program was developed. Mail surveys were distributed to 1,263 U. S. jails with a capacity of 5O or more detainees to ascertain the presence or absence of diversion programs. Telephone interviews with samples of respondents and nonrespondents to the mail survey yielded additional information about the programs' operation, funding, staffing, and directors' perceptions of their effectiveness. Results: Information obtained from the mail and telephone surveys indicated that only 52 U. S. jails with a capacity of 5O or more detainees had formal mental health diversion programs that fit the definition developed by the authors. Programs in larger jails servedfewer viokntfelons than did those in smaller jails. Threefourths of the programs were located in mental health agencies. Two-thirds of program directors considered theprograms to be moderately or very effective. Conclusions: Only a small number of U. S. jails have diversion programs for mentally ill detainees, and objective data on their effectiveness are lacking. Systematic evaluations are needed to determine what types of programs work best for which types of detainees.