Purpose: To evaluate pathways through the criminal justice system for 63 prisoners under the care of prison mental health services. Results: Only a small number (3%) were acutely mentally ill on reception to prison, and this may reflect the successful operation of liaison and diversion services at earlier stages in the pathway. However, a third (33%) went onto display acute symptoms at later stages. Cases displaying suicide risk at arrest, with a history of in-patient care, were at increased risk of acute deterioration in the first weeks of imprisonment, with a general lack of health assessments for these cases prior to their imprisonment. Inconsistencies in the transfer of mental health information to health files may result in at-risk cases being overlooked, and a lack of standardisation at the court stage resulted in difficulties determining onward service provision and outcomes. Foreign national prisoners were under-represented in the sample. Conclusions: Greater consistency in access to pre-prison health services in the criminal justice system is needed, especially for those with pre-existing vulnerabilities, and it may have a role in preventing subsequent deterioration. A single system for health information flow across the whole pathway would be beneficial. Summary points: 1. Only 3% of cases were acutely mentally ill at prison reception. 2. Cases identified at arrest as presenting a high risk of suicide, with a history of in-patient care, may be at increased risk of acute deterioration in the first weeks of imprisonment; but they were over-looked for assessment prior to prison. 3. Inconsistencies in the transfer of mental health information into prison health files may result in overlooked at-risk cases.