School children appear to increase their vocabularies by thousands of words per year. Many have hypothesized that a large proportion of this growth occurs through incidental learning from written context. However, experimental research has until now failed to provide unequivocal support of this hypothesis. The present study attempted to determine whether students do acquire measurable knowledge about unfamiliar words while reading natural text. Fifty-seven eighth-grade students of average and above average reading ability read either an expository or a narrative text about 1,000 words in length. After reading, subjects completed two vocabulary assessment tasks on 15 target words from each passage (thus serving as controls for the passage not read), an individual interview and a multiple-choice test, both designed to tap 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 lead us to believe that incidental learning from context accounts for a substantial proportion of the vocabulary growth that occurs during the school years.