We examined the association between the duration of untreated psychosis (DUP) and outcome for patients with delusional infestation (DI). This multi-centre international study included consecutive 211 patients. Illness severity was evaluated at first presentation and outcome was measured with the Clinical Global Impression scale (CGI) at base line and follow-up. A regression analysis showed a clear clinical and statistically significant association between shorter duration of untreated psychosis and better outcome at follow up. Patients with a DUP of less than 1 year showed a CGI-S change from 5.37 to 2.07; those with a DUP of 1-5 years a change from 5.48 to 2.59, and those with a DUP of above 5 years a change from 5.59 to 3.37. This difference of 1.1 CGI points between the groups resembles a clinically relevant difference in patient outcome Our results suggest that longer duration of untreated psychosis in patients with DI is associated with significantly less favourable clinical outcomes.