In patients with early psychosis, medication non-adherence is associated with more frequent relapse. Observational studies have reported that up to 60% of patients fail to take medication as prescribed. This study aimed to establish the effectiveness of adherence therapy (AT) training for two multidisciplinary early intervention in psychosis (EIIP) teams in preventing relapse in their patients. This intervention involved six 1-day team AT training sessions delivered monthly over a 6-month period. Participants were patients with early psychosis who were on the caseload of the EIIP teams during the study period. A mirror-image design was used, comparing clinician ratings of relapse in the year preceding training (year 0) and the subsequent year (year 1). Results showed that in year 0, the mean number of relapses was 0.96 (standard deviation (SD) = 1.10). During year 1, relapses reduced significantly (P < 0.01) to 0.34 (SD = 0.64). No unexpected effects of training were reported. A thematic analysis of staff views of training, and service users' views of their care received from EIIP teams, was also completed. Challenges in faithfully incorporating AT skills into practice were reported. In conclusion, AT training coincided with reductions in relapse rates in patients receiving services from an EIIP team, but no other changes in outcomes were detected.