In this preregistered study, we used latent change score models to address two research aims: (1) whether preschool-aged children's language gains, over a year of early childhood education, were associated with later performance on state-mandated, literacy-focused kindergarten readiness and Grade 3 reading achievement assessments; and (2) whether gains in language, a more complex skill, predicted these outcomes after controlling for more basic emergent literacy skills. There were 724 participating children (mean = 57 months; 51% male; 76% White, 12% Black, 6% multiple races, and 5% Hispanic or Latino). We found that language gains significantly predicted kindergarten readiness when estimated in isolation (effect = 0.24 SDs, p < .001), but not when gains in letter knowledge and phonological awareness were also included.