Mental health care providers in communities and State hospitals have witnessed an increase in the number of patients who have a history of correctional incarcerations. Most mentally ill offenders serve time in county jails for misdemeanors and public nuisance charges, and local jails have replaced State hospitals as the repository for increasing numbers of this population. Factors affecting the likelihood that an individual's unusual or deviant behavior will be dealt with by the criminal justice system rather than the mental health system include the unavailability of long-term hospitalization in a State facility for the chronic mentally ill, the lack of adequate support systems for the mentally ill in the community, more formal and rigid criteria for civil commitment, and expectations that police deal with deviant behavior more quickly and efficiently than the mental health system does. The authors define mentally ill offenders and discuss the treatment of mentally ill offenders in prisons and the return of these offenders to the community. Legal and treatment needs of the mentally ill offender population during incarceration are examined, and obstacles to caring for them after their eventual release from prison are noted. The authors recommend that mentally ill offenders be afforded the same level of mental health care available to mentally ill residents in other institutions and in the community. Postrelease supervision and coordination between mental health and correctional authorities are viewed as central issues in reintegrating mentally ill offenders into the community. 106 references.