Background: Children with language comprehension difficulties are at risk of educational and social problems, which in turn impede employment prospects in adulthood. However, few randomized trials have examined how such problems can be ameliorated during the preschool years. Methods: We conducted a cluster randomized trial in 148 preschool classrooms. Our intervention targeted language comprehension skills and lasted 1 year and 1 month, with five blocks of 6 weeks and intervention three times per week (about 75 min per week). Effects were assessed on a range of measures of language performance. Results: Immediately after the intervention, there were moderate effects on both near, intermediate and distal measures of language performance. At delayed follow-up (7 months after the intervention), these reliable effects remained for the distal measures. Conclusions: It is possible to intervene in classroom settings to improve the language comprehension skills of children with language difficulties. However, it appears that such interventions need to be intensive and prolonged.