Bluebird House is a mixed-gender adolescent medium-secure forensic unit. There is a paucity of data available about this unique patient group. We aimed to explore the demographic, clinical and personality characteristics of the first 100 patients admitted to a mixed-gender adolescent medium-secure forensic unit. We used a retrospective cross-sectional study of the first 100 consecutive patients admitted to Bluebird House. Demographic and clinical data were collected from the medical notes including the Millon Adolescent Clinical Inventory. The first 100 young people admitted to an adolescent medium-secure unit have had very high incident rates, particularly amongst the females. Incident rate was correlated with a history of sexual assault in female patients. The males were more likely to have been referred from custody and more likely to have a diagnosis of psychosis or autism. We describe similarities and differences to the first 100 admissions admitted to the first secure psychiatric unit for adolescents in Manchester over 20 years ago.