A clinical staging model of psychotic disorders based on a long-term follow-up of first-admission psychosis: A validation study

•We developed a novel clinical staging model of psychosis based on the long-term course and outcome of 243 first-episode psychosis followed-up for a mean of 21 years.•The model's validity was assessed by means of 22 baseline predictors, 23 construct-related variables and 31 outcome variables.•The model showed good inter-rater reliability and sound construct, outcome and predictive validity.•Eight baseline variables independently accounted for 49.9% of the variance of staging.