Compared with adult onset psychosis, adolescent psychosis has been associated with poorer outcomes in terms of social and cognitive functioning and negative symptoms (Boeing et al., 2007; Mayoral et al., 2008). Young people experiencing first episode psychosis have developmental needs that frequently pre-date and are compounded by psychosis onset (Gumley & Schwannauer, 2006; Harrop & Trower, 2001). There is a lack of published studies of adolescent onset psychosis and further information is needed so that developmentally appropriate interventions can be developed (Schimmelmann & Schultze-Lutter, 2012). We report an observational naturalistic cohort study of an adolescent specific service, the Early Psychosis Support service (EPSS). We examined baseline demographic and clinical variables, treatments outcomes, and predictors of outcome for this population. The mean age of our sample was 16.3 years. Median duration of untreated illness (DUI) was 88 weeks, and median duration of untreated psychosis (DUP) was 16 weeks. We found significant improvements in positive symptoms, negative symptoms, disorganisation, excitement, emotional distress and depression from 0 to 12 months. We found that baseline positive symptoms and DUI significantly predicted positive symptoms at 12 months and only negative symptoms at baseline predicted 12-month negative symptoms. Our finding that specialist early intervention for adolescents experiencing psychosis is effective suggests that such treatment should be routinely offered in line with existing clinical guidelines. Our finding that DUI is predictive of poorer outcome at 12 months suggests that even earlier intervention from a specialist team may further improve treatment outcomes.