BackgroundStaff training in positive behaviour support (PBS) is a widespread treatment approach for challenging behaviour in adults with intellectual disability.AimsTo evaluate whether such training is clinically effective in reducing challenging behaviour during routine care (trial registration: NCT01680276).MethodWe carried out a multicentre, cluster randomised controlled trial involving 23 community intellectual disability services in England, randomly allocated to manual-assisted staff training in PBS (n = 11) or treatment as usual (TAU, n = 12). Data were collected from 246 adult participants.ResultsNo treatment effects were found for the primary outcome (challenging behaviour over 12 months, adjusted mean difference = −2.14, 95% CI: −8.79, 4.51) or secondary outcomes.ConclusionsStaff training in PBS, as applied in this study, did not reduce challenging behaviour. Further research should tackle implementation issues and endeavour to identify other interventions that can reduce challenging behaviour.Declaration of interestNone.