OBJECTIVE Understanding factors that contribute to treatment delay would inform early detection and intervention strategies in psychotic disorders. However, existing data were mixed and primarily conducted among early-onset young patients. We examined duration of untreated psychosis (DUP) and its clinical and sociodemographic correlates in a large cohort of adult-onset patients with psychosis. METHODS A total of 360 patients with first-onset psychosis aged 26-55 years were recruited consecutively as part of a controlled study of an early psychosis intervention service in Hong Kong Chinese. Demographic, sociodemographic and clinical characteristics relating to DUP were assessed within 4 months of onset. RESULTS The population had a mean onset age of 36.6 years (SD = 8.7). The mean and median DUP were 515 days (SD = 1091) and 93 days (inter-quartile range from 20 to 382.3), respectively. Multivariate regression analysis suggested that insidious mode of onset, hospitalization, a diagnosis of schizophrenia, poorer insight and younger age at onset significantly prolonged DUP. DUP was not related to premorbid functioning, family involvement during help seeking and living alone. CONCLUSIONS The initial period of untreated psychosis is determined by multiple factors. Whether family involvement is considered a kind of social support in shortening or prolonging DUP needs further examination. Local early intervention program for psychosis should take reference from these findings when formulating personalized plans to reduce delay.