Mental health courts are growing in popularity as a form of jail diversion for justice system-involved people with serious mental illness. This is the first prospective multisite study on mental health courts with treatment and control groups. To determine if participation in a mental health court is associated with more favorable criminal justice outcomes than processing through the regular criminal court system and to identify defendants for whom mental health courts produce the most favorable criminal justice outcomes. Longitudinal study. Four mental health courts in San Francisco County, CA, Santa Clara County, CA, Hennepin County (Minneapolis), MN, and Marion County (Indianapolis), IN. A total 447 persons in the mental health court (MHC) and 600 treatment-as-usual (TAU) controls. Eighteen months of pre-entry and postentry data for 4 jurisdictions. All subjects were interviewed at baseline, and 70% were interviewed at 6 months. Objective outcome data were obtained on all subjects from Federal Bureau of Investigation arrest records, jails, prisons, and community treatment providers. Annualized rearrest rates, number of rearrests, and postentry incarceration days. The MHC and TAU samples are similar on the major outcome measures in the pre-entry 18-month period. In the 18 months following treatment, defined as entry into mental health court, the MHC group has a lower annualized rearrest rate, fewer post-18-month arrests, and fewer post-18-month incarceration days than the TAU group. The MHC graduates had lower rearrest rates than participants whose participation was terminated both during MHC supervision and after supervision ended. Factors associated with better outcomes among the MHC participants include lower pre-18-month arrests and incarceration days, treatment at baseline, not using illegal substances, and a diagnosis of bipolar disorder rather than schizophrenia or depression. Mental health courts meet the public safety objectives of lowering posttreatment arrest rates and days of incarceration. Both clinical and criminal justice factors are associated with better public safety outcomes for MHC participants.