This study involves the collaboration of researchers with 24 teachers in a 3-year intervention study aimed at investigating and improving vocabulary knowledge and reading comprehension of fourth and fifth grade students for whom English is a second language. Cross-sectional and longitudinal data were collected on various aspects of vocabulary knowledge and reading comprehension of English language learners and English-only peers. In year one, the vocabulary knowledge of all students was tested, and a large gap between the English language learners and English-only students was discovered. A set of lessons was subsequently developed that was hoped would reduce the gap between the groups. In year two, the lessons (20-40 minutes long, offered 4 days a week for 12 to 15 weeks) were given to fourth graders at sites in three states. It was found that after 12 weeks of supplemental instruction the breadth of vocabulary knowledge was measurably improved, but reading comprehension was not noticeably improved. In year three, the intervention was again administered to the same students, now in fifth grade, and to a new group of fifth graders. Students receiving this extra instruction outperformed comparable students on measures of breadth and depth of vocabulary knowledge and reading comprehension. Students who received both years of instruction had a measurable boost in vocabulary knowledge, but not a significant difference in reading comprehension from those receiving only one year. Overall, English language learners in 2 years closed the gap with native speakers by about 50 percent--from about one to 0.5 standard deviations. It was concluded, therefore, that the extra instruction was quite effective. Extensive references and tables of quantitative data are included. (Contains 19 references.) (KFT)