OBJECTIVE: To determine the benefits of a low intensity parent-toddler language promotion programme delivered to toddlers identified as slow to talk on screening in universal services. DESIGN: Cluster randomised trial nested in a population based survey. SETTING: Three local government areas in Melbourne, Australia. PARTICIPANTS: Parents attending 12 month well child checks over a six month period completed a baseline questionnaire. At 18 months, children at or below the 20th centile on an expressive vocabulary checklist entered the trial. INTERVENTION: Maternal and child health centres (clusters) were randomly allocated to intervention (modified "You Make the Difference" programme over six weekly sessions) or control ("usual care") arms. MAIN OUTCOME MEASURES: The primary outcome was expressive language (Preschool Language Scale-4) at 2 and 3 years; secondary outcomes were receptive language at 2 and 3 years, vocabulary checklist raw score at 2 and 3 years, Expressive Vocabulary Test at 3 years, and Child Behavior Checklist/1.5-5 raw score at 2 and 3 years. RESULTS: 1217 parents completed the baseline survey; 1138 (93.5%) completed the 18 month checklist, when 301 (26.4%) children had vocabulary scores at or below the 20th centile and were randomised (158 intervention, 143 control). 115 (73%) intervention parents attended at least one session (mean 4.5 sessions), and most reported high satisfaction with the programme. Interim outcomes at age 2 years were similar in the two groups. Similarly, at age 3 years, adjusted mean differences (intervention-control) were -2.4 (95% confidence interval -6.2 to 1.4; P=0.21) for expressive language; -0.3 (-4.2 to 3.7; P=0.90) for receptive language; 4.1 (-2.3 to 10.6; P=0.21) for vocabulary checklist; -0.5 (-4.4 to 3.4; P=0.80) for Expressive Vocabulary Test; -0.1 (-1.6 to 1.4; P=0.86) for externalising behaviour problems; and -0.1 (-1.3 to 1.2; P=0. 92) for internalising behaviour problems. CONCLUSION: This community based programme targeting slow to talk toddlers was feasible and acceptable, but little evidence was found that it improved language or behaviour either immediately or at age 3 years. TRIAL REGISTRATION: Current Controlled Trials ISRCTN20953675.