Objective: The present study aimed to assess the mental and physical health of children held within a British immigration detention center. Method: A total of 24 detained children (aged 3 months to 17 years) were assessed with their parents or carer after being referred by a registered legal charity. Thirteen were seen by a pediatrician alone, 4 by a psychologist alone, and 7 by both professions using semi-structured clinical interviews. The psychologist also used standardized self-report questionnaires to measure psychopathology. Results: During the psychological assessment of 11 children, 8 met criteria for psychiatric "caseness" on the Strengths and Difficulties Questionnaire. All 11 reported symptoms of depression and anxiety. Sleep problems, somatic complaints, poor appetite, emotional symptoms, and behavioral difficulties were common. Symptoms of global distress were also reported by all 9 parents. According to pediatric assessment 8 out of 20 children had lost weight. Six had missed health appointments and 2 were taken to hospital. Nutritional, developmental, educational, and child protection concerns were raised. Conclusions: Detained children were found to be experiencing mental and physical health difficulties of recent onset, which appeared to be related to the detention experience. These findings support previous Australian studies demonstrating that detention is not in the best interest of the child. It suggests that current UK policies regarding the detention of children for purposes of immigration control should be re-examined. Further research in the area is required. Practice implications: Although high levels of mental and physical health problems, as well as child protection concerns were detected, detained families had very limited access to appropriate assessment, support or treatment. The traumatic experience of detention itself also has implications for the sizeable proportion of psychologically distressed children who are eventually released from detention and expected to successfully reintegrate into British society; while those children who are deported are returned with increased vulnerability to future stressors. (Contains 3 tables.)