The study findings show that Santa Fe police have daily encounters with mentally ill and substance-abusing individuals, as shown by data on the use of protective custody and mental-health holds over time. A review of assessments and hold histories of individuals detained on holds (April 1998 to May 1999) showed that the majority of these individuals had multiple problems, including alcohol abuse, drug use, serious mental disorders, and involvement in criminal activity. This population places a heavy financial burden on the local criminal justice system, particularly in incarceration costs. Individuals detained on holds were significantly more likely than the general arrest population to have additional police contacts, which translates into additional costs to the criminal justice system. These findings support prior research in showing that the criminal justice system is unprepared financially and in treatment resources to deal with the needs of the mentally ill and substance-abusing population. The responsibility for change falls on a wide range of agencies and policymakers, both within and outside the criminal justice system. The study collected data on aggregate monthly arrest and protective custody or mental-health holds for April 1, 1997, to January 31, 2000. Data were also collected on demographics, assessments, criminal histories (arrest and confinement), prior hold history, and behavior during a 1-year followup period for a random sample of individuals detained on protective custody or mental-health holds. The same types of data were collected for a random sample of persons arrested on criminal charges. 5 tables, 14 notes, and 73 references