We have previously shown short-term benefits to phonology, letter knowledge, and possibly expressive language from systematically ascertaining language delay at age 4 years followed by the Language for Learning intervention. Here, we report the trial's definitive 6-year outcomes.Randomized trial nested in a population-based ascertainment. Children with language scores >1.25 SD below the mean at age 4 were randomized, with intervention children receiving 18 1-hour home-based therapy sessions. Primary outcome was receptive/expressive language. Secondary outcomes were phonological, receptive vocabulary, literacy, and narrative skills; parent-reported pragmatic language, behavior, and health-related quality of life; costs of intervention; and health service use. For intention-to-treat analyses, trial arms were compared using linear regression models.Of 1464 children assessed at age 4, 266 were eligible and 200 randomized; 90% and 82% of intervention and control children were retained respectively. By age 6, mean language scores had normalized, but there was little evidence of a treatment effect for receptive (adjusted mean difference 2.3; 95% confidence interval [CI] -1.2 to 5.7; P = .20) or expressive (0.8; 95% CI -1.6 to 3.2; P = .49) language. Of the secondary outcomes, only phonological awareness skills (effect size 0.36; 95% CI 0.08-0.65; P = .01) showed benefit. Costs were higher for intervention families (mean difference AU$4276; 95% CI: $3424 to $5128).Population-based intervention targeting 4-year-old language delay was feasible but did not have lasting impacts on language, possibly reflecting resolution in both groups. Long-term literacy benefits remain possible but must be weighed against its cost.