Research has revealed high levels of mental health needs in young offenders but many studies have been small, focusing on specific populations. To evaluate the mental health and psychosocial needs of a nationally representative sample of juvenile offenders in England and Wales, including female offenders and those from Black and minority ethnic groups. A cross-sectional survey of 301 young offenders, 151 in custody and 150 in the community, was conducted in six geographically representative areas across England and Wales. Each young person was interviewed to obtain demographic information, mental health and social needs, and psychometric data. Young offenders were found to have high levels of needs in a number of different areas including mental health (31%), education/work (36%) and social relationships (48%). Young offenders in the community had significantly more needs than those in secure care and needs were often unmet. One in five young offenders was also identified as having a learning disability (IQ<70). Needs for young offenders were high but often unmet. This emphasises the importance of structured needs assessment within custody and community settings in conjunction with a care programme approach that improves continuity of care.