To test the hypothesis that a large proportion of school children's vocabulary growth occurs through incidental learning from written context, a study was conducted using 57 eighth grade students of average and above average reading ability. The subjects read a 1,000 word expository or narrative text. After reading, subjects completed two vocabulary assessment tasks on 15 target words from each passage, an individual interview, and a multiple choice test, both designed to test partial knowledge of word meanings. Results of within-subject, hierarchical regression analyses showed small but statistically reliable gains in word knowledge from context. Tentative extrapolations from the results and current estimates of the volume of children's reading resulted in the conclusion that incidental learning from context accounts for a substantial proportion of the vocabulary growth that occurs during the school years. (Author/CRH)