Background: There is highly replicated positive correlation between longer duration of untreated psychosis and poorer outcome. Aims: To study the effect of early intervention in first psychosis on one-year outcome using an historical quasi-experimental design. Method: We compare the outcome of two samples of first-episode psychosis from the same healthcare district at different time periods. The historical control sample was assessed during 1993-1994, before the establishment of a system for early detection of psychosis. The experimental sample is the early detection sample in the Early Treatment and Intervention in Psychosis study assessed during 1997-2000. Results: At 1-year follow-up, the early detection group was younger, had a smaller fraction of individuals with schizophrenia, had less severe negative and general symptoms and had more friends in the past year than the historical control group. No differences were found in clinical course (remission, relapse, continuously psychotic) or positive symptoms, but more patients in the early detection sample were treated as outpatients without hospitalisation. Conclusions: Early detection of schizophrenia in one healthcare sector is associated with less severe deterioration at 1 year.