Objective To investigate changes in mental health and other needs, as well as clinical and diagnostic ‘caseness’, in a sample of adolescents over a 6-month period following entry into a Young Offenders Institution in the UK.

Design Prospective cohort study.

Setting One Young Offenders Institution between November 2006 and August 2009.

Participants 219 male adolescents aged 15–18 years (M=16.56; SD=0.6) were assessed at baseline (median=4; range 0–26 days following reception into custody) on the Salford Needs Assessment Schedule for Adolescents (SNASA) and Kiddie Schedule for Affective Disorders and Schizophrenia (K-SADS). Participants were then reassessed at 3-month and 6-month postbaseline to document any change in mental health.

Results Of the initial baseline sample, 132 were still in the study at 3-month postbaseline and 63 were still available for assessment at 6 months. There were no differences between those who were not available for assessment at the three key stages in terms of demographic and criminological data. Over time there was a general improvement in mental health. While the proportion of participants with a mental health need (SNASA) did not change over time, symptom severity as measured by the SNASA did reduce significantly. When we assessed diagnostic ‘caseness’ using the K-SADS, three young people showed significant mental health deterioration.

Conclusions In line with previous studies, we found that symptoms in prison generally improved over time. Prison may provide an opportunity for young people previously leading chaotic lifestyles to settle into a stable routine and engage with services; however, it is unclear if these would be maintained either within the prison or on release into the community.