Although criminal justice involvement among persons with severe mental illness is a much discussed topic, few large-scale studies systematically describe the patterns and prevalence of arrest in this population. This study examined rates, patterns, offenses, and sociodemographic correlates of arrest in a large cohort of mental health service recipients. The arrest records of 13,816 individuals receiving services from the Massachusetts Department of Mental Health from 1991 to 1992 were examined over roughly a ten-year period. Bivariate relationships between sociodemographic factors and arrest were also examined. About 28 percent of the cohort experienced at least one arrest. The most common charges were crimes against public order followed by serious violent offenses and minor property crime. The number of arrests per individual ranged from one to 71. Five percent of arrestees (roughly 1.5 percent of the cohort) accounted for roughly 17 percent of arrests. The proportion of men arrested was double that of women. Persons 18 to 25 years of age had a 50 percent chance of at least one arrest. This rate declined with age but did so unevenly across offense types. The likelihood of arrest appeared substantial among persons with severe mental illness, but the bulk of offending appeared concentrated in a small group of persons and among persons with sociodemographic features similar to those of offenders in the general population. Data such as these could provide a platform for designing jail diversion and other services to reduce both initial and repeat offending among persons with serious mental illness.(Psychiatric