Longer duration of untreated psychosis (DUP) in adult patients with first-episode psychosis (FEP) has been associated with poor clinical and social outcomes. We aimed to estimate the influence of DUP on outcome at 2-year follow-up in subjects with an early-onset (less than 18years of age) FEP of less than 6months' duration. A total of 80 subjects (31.3% females, mean age 16.0±1.8years) were enrolled in the study. The influence of DUP on outcome was estimated using multiple regression models (two linear models for influence of DUP on the C-GAF at 2years and C-GAF change through the follow-up period, and a logistic model for influence of DUP on 41 PANSS remission at 2 years in schizophrenia patients (n=47)). Mean DUP was 65.3±54.7days. Median DUP was 49.5days. For the whole sample (n=80), DUP was the only variable significantly related to C-GAF score at 2-year follow-up (Beta=-0.13, p<0.01), while DUP and premorbid adjustment (Beta=-0.01, p<0.01; and Beta=-0.09, p=0.04, respectively) were the only variables significantly related to C-GAF change. In schizophrenia patients, DUP predicted both C-GAF score at 2years and C-GAF change, while in patients with affective psychosis (n=22), DUP was unrelated to outcome. Lower baseline C-GAF score (OR=0.91, p<0.01) and shorter DUP (OR=0.98, p=<0.01) were the only variables that significantly predicted clinical remission in schizophrenia patients. In conclusion, longer DUP was associated with lower C-GAF at 2years, less increase in C-GAF, and lower rates of clinical remission in early-onset FEP. Our findings support the importance of early detection programs, which help shorten DUP.