Background: Information on determinants of duration of untreated psychosis (DUP) is still needed to inform campaigns targeting people with first episode psychosis (FEP). This nation-wide study analysed the association between demographic factors (age, sex, ethnicity, marital status, and geographic area), premorbid and illness-related factors (global functional level, substance misuse, and contact to police), healthcare factors (referral source and first FEP contact) and DUP. Method: The study population of 1266 patients aged 15-25. years diagnosed with FEP (ICD10 F20.0-F20.99) was drawn from the Danish National Indicator Project during 2009-2011. The study population was combined with data from national administrative registers. A multinomial regression model was estimated to analyse the impact of demographic, premorbid and illness-related, and healthcare factors on DUP. Results: One third of the population had a DUP below 6. months. DUP longer than 12. months was associated with older age at onset, being female, having cannabis misuse, and living in peripheral municipalities. Being charged by the criminal authorities during one year before FEP was associated with a DUP over 6. months. Conclusion: DUP is related to a number of demographic, premorbid and healthcare factors. These findings suggest that future information campaigns should focus on increasing the awareness of early signs of psychosis not only among mental health professionals but also other professionals in contact with adolescents such as the police. It may also be useful to consider how to target information campaigns towards persons living in peripheral areas.