The present article reports results of a real-world effectiveness trial conducted in Denmark with six thousand four hundred eighty-three 3- to 6-year-olds designed to improve children's language and preliteracy skills. Children in 144 child cares were assigned to a control condition or one of three planned variations of a 20-week storybook-based intervention: a base intervention and two enhanced versions featuring extended professional development for educators or a home-based program for parents. Pre- to posttest comparisons revealed a significant impact of all three interventions for preliteracy skills (= .21-.27) but not language skills (d = .04-.16), with little differentiation among the three variations. Fidelity, indexed by number of lessons delivered, was a significant predictor of most outcomes. Implications for real-world research and practice are considered. (author abstract)