Background: This study examines the role of decoding and vocabulary skills as longitudinal predictors of reading comprehension in young first (L1) and second (L2) language learners. Methods: Two-group latent growth models were used to assess differences in growth and predictions of growth between the 198 L1 and 90 L2 language learners. Results: L1 learners had better initial reading comprehension skills and faster growth in these skills over time. Individual differences in decoding and vocabulary predicted initial reading comprehension skills, but only vocabulary predicted the subsequent growth of reading comprehension skills. Vocabulary seemed to be a stronger predictor of growth in reading comprehension among the L2 learners than among the L1 learners. Conclusions: Vocabulary appears to be a critical predictor of the early development of reading comprehension skills in both L1 and L2 learners. The limitations in vocabulary skills in the L2 learners seemed sufficient to explain their lag in developing reading comprehension skills, and this suggests that oral vocabulary training should be given a high priority in this group.