The authors carried out 24-hour observations for 21 days in the custody suites of seven different metropolitan police stations in England. They collected detailed information on characteristics of detainees and how they were processed. In addition, psychiatrists involved in court-based assessment schemes in London pinpointed defendants they considered to be suffering from a major mental illness. Custody records for these individuals were examined, and arresting officers were interviewed. It was found that 37 of 2,721 people detained by the police (1.4 percent) were considered to be actively mentally ill and a further 20 (0.7 percent) were classified as possibly ill. Of the sample, 16 (0.6 percent) were known to have had hospital admissions for psychotic illnesses, although they displayed no current symptoms; 8 were mentally handicapped; and a further 23 detainees showed some signs of being below average intelligence. Appropriate adults were present in 10 interviews where police officers knew or suspected the detainee was mentally disordered. In a further three cases, the authors thought an appropriate adult should have been present but the detainee was judged fit for interview by a doctor. Mentally ill offenders whose cases came to court were more likely than the detainee sample to have been arrested for a violent offense and to have been violent at arrest. Persistent offending and failure to answer a warrant were also associated with a case coming to court. The importance of balancing the needs of mentally ill offenders against the needs of society is addressed. 5 references, 1 table, and 1 figure