Background: Young people with self-experienced cognitive thought and perception deficits (basic symptoms) may present with an early initial prodromal state (EIPS) of psychosis in which most of the disability and neurobiological deficits of schizophrenia have not yet occurred. Aims: To investigate the effects of an integrated psychological intervention (IPI), combining individual cognitive-behavioural therapy, group skills training, cognitive remediation and multifamily psychoeducation, on the prevention of psychosis in the EIPS. Method: A randomised controlled, multicentre, parallel group trial of 12 months of IPI v. supportive counselling (trial registration number: NCT00204087). Primary outcome was progression to psychosis at 12- and 24-month follow-up. Results: A total of 128 help-seeking out-patients in an EIPS were randomised. Integrated psychological intervention was superior to supportive counselling in preventing progression to psychosis at 12-month follow-up (3.2% v. 16.9%; P = 0.008) and at 24-month follow-up (6.3% v. 20.0%; P = 0.019). Conclusions: Integrated psychological intervention appears effective in delaying the onset of psychosis over a 24-month time period in people in an EIPS. Declaration of interest: None.