In 1939 Penrose explained the correlation between crime and mental illness in terms of an administrative rather than an etiological link. Penrose's theory encompasses the idea that social standards for defining aberrant behaviour change as do the administrative facilities designed to control such behaviour. Social groups cope with individuals who display undesirable behaviour in one of two ways. They define the behaviour as incompetent and invoke the mental health system or they define the behaviour as criminal and employ the jail. Penrose documented an inverse relationship between these facilities whereby the population of mental hospitals increases as the population of jails decreases, and vice versa, depending on the prevailing laws, funding or stage of development of either system. The actual size of the population requiring institutional care, however, remains stable forcing individuals to transmigrate from one system to the other in order to obtain institutional support. Over the past two decades, increasing numbers of mentally ill have found their way into jails, however, little systematic evidence has been produced to document the process followed by these individuals as they move through the justice-correctional system, or how their psychiatric needs are met. The authors have conducted a prospective longitudinal investigation to document the transmigration of mental patients through the criminal justice system and have followed a cohort of individuals arrested in the City of Calgary during the month of October, 1984. Arresting officers were asked to complete a special study form to identify individuals who, in their perception, displayed mentally disordered behaviour during the arrest.(ABSTRACT TRUNCATED AT 250 WORDS)