The police are the gatekeepers to the criminal justice system (CJS). It is agreed that where possible mentally ill people should be diverted to psychiatric care. The purpose of this investigation was to determine what factors were associated with the entry of obviously mentally ill people into the CJS.

The population of all detainees arrested in seven London police stations was observed to determine the presence of obvious mental illness. All detainees regarded as ill or known to have significant psychiatric histories were followed through the CJS to court disposal. Defendants diagnosed as psychotic by court liaison psychiatrists were identified and the circumstances of their arrest and detention examined.

Of all police detainees observed, 1.4% were found to be acutely ill. Diversion of obviously ill detainees was common practice. The factor most strongly associated with entry to the CJS was the presence of violence at time of arrest. Other factors were the persistence of (petty) offending and the operation of court warrants issued as a result of the person's failure to appear at court when required to do so.

Both police and courts are well aware of the inappropriateness of custody for acutely ill people and efforts are made to divert such people out of the CJS. Persistent petty offenders are often being recycled from the street to police station to court and back to the street without the benefit of care. A dedicated facility is recommended in central London to meet this need.