Purpose: We investigated the roles of leisure reading and word reading ability in vocabulary and reading comprehension development in 598 adolescents at ages 10, 11, and 12 (285 girls, 313 boys). Method: Structural equation modeling was used to test whether word reading was associated with vocabulary and reading comprehension: a) directly; b) indirectly via leisure reading; or c) both. Results: We found both direct and indirect effects of word reading on vocabulary: word reading ability directly predicted outcomes, and also predicted the amount of leisure reading, which in turn predicted vocabulary. For reading comprehension we observed direct but not indirect effects of word reading. As expected, vocabulary and reading comprehension outcomes were strongly correlated. Conclusion: Our findings demonstrate the direct effect of word reading ability in predicting vocabulary and reading comprehension, and reveal a crucial mediating role of leisure reading in the development of vocabulary.