A study investigated the relationships among vocabulary size, depth of vocabulary knowledge, and reading comprehension in English as a second language (ESL). Subjects were 74 native Chinese- and Korean-speaking students in intensive academic ESL courses in two Ontario universities. They were administered questionnaires to gather demographic and language background information, and four English language tests: vocabulary size; reading comprehension; depth of vocabulary knowledge; and morphological knowledge. Results support the hypotheses that scores on ESL vocabulary size, depth of vocabulary knowledge, and reading comprehension would correlate highly with each other, and that for learners with an English vocabulary size of over 3,000, scores on depth of vocabulary knowledge would make a significant and unique contribution to prediction of reading comprehension. Implications for second language education are considered. (Contains 30 references.) (MSE)