Purpose: Individual differences in vocabulary development may affect academic or social opportunities. It has been proposed that individual differences in word reading could affect the rate of vocabulary growth, mediated by the amount of reading experience, a process referred to as a "Matthew effect" (Stanovich, 1986). Method: In the current study, assessments of written word-reading skills in the 4th grade and oral vocabulary knowledge collected in kindergarten and in the 4th, 8th, and 10th grades from a large epidemiologically based sample (n = 485) allowed a test of the relationship of early word-reading skills and the subsequent rate of vocabulary growth. Results: Consistent with the hypothesis, multilevel modeling revealed the rate of vocabulary growth after the 4th grade to be significantly related to 4th-grade word reading after controlling for kindergarten vocabulary level, that is, above average readers experienced a higher rate of vocabulary growth than did average readers. Conclusions: Vocabulary growth rate differences accumulated over time such that the effect on vocabulary size was large.