Summary Background Results of small trials suggest that early interventions for social communication are effective for the treatment of autism in children. We therefore investigated the efficacy of such an intervention in a larger trial. Methods Children with core autism (aged 2 years to 4 years and 11 months) were randomly assigned in a one-to-one ratio to a parent-mediated communication-focused (Preschool Autism Communication Trial PACT) intervention or treatment as usual at three specialist centres in the UK. Those assigned to PACT were also given treatment as usual. Randomisation was by use of minimisation of probability in the marginal distribution of treatment centre, age (≤42 months or >42 months), and autism severity (Autism Diagnostic Observation Schedule-Generic ADOS-G algorithm score 12–17 or 18–24). Primary outcome was severity of autism symptoms (a total score of social communication algorithm items from ADOS-G, higher score indicating greater severity) at 13 months. Complementary secondary outcomes were measures of parent-child interaction, child language, and adaptive functioning in school. Analysis was by intention to treat. This study is registered as an International Standard Randomised Controlled Trial , number ISRCTN58133827. Results 152 children were recruited. 77 were assigned to PACT (London n=26, Manchester n=26, and Newcastle n=25); and 75 to treatment as usual (London n=26, Manchester n=26, and Newcastle n=23). At the 13-month endpoint, the severity of symptoms was reduced by 3·9 points (SD 4·7) on the ADOS-G algorithm in the group assigned to PACT, and 2·9 (3·9) in the group assigned to treatment as usual, representing a between-group effect size of −0·24 (95% CI −0·59 to 0·11), after adjustment for centre, sex, socioeconomic status, age, and verbal and non-verbal abilities. Treatment effect was positive for parental synchronous response to child (1·22, 0·85 to 1·59), child initiations with parent (0·41, 0·08 to 0·74), and for parent-child shared attention (0·33, −0·02 to 0·68). Effects on directly assessed language and adaptive functioning in school were small. Interpretation On the basis of our findings, we cannot recommend the addition of the PACT intervention to treatment as usual for the reduction of autism symptoms; however, a clear benefit was noted for parent-child dyadic social communication. Funding UK Medical Research Council, and UK Department for Children, Schools and Families.