This report contends that limited vocabulary knowledge is the principal cause of reading dysfunction for a large percentage of students whose progress in learning to read appears normal during the first two or three years of reading instruction, but who begin to fall behind starting somewhere between grades 3 and 7. The report addresses the reasons why this causal relationship has not achieved more prominence in the literature, including (1) disparate findings regarding the effects of vocabulary instruction on both vocabulary knowledge and on reading comprehension; (2) the reciprocal relationship between vocabulary and reading; and (3) the absence of clear, broad-based data demonstrating a high correlation between the vocabulary size of young readers and their subsequent reading development. Suggestions for remedying the problem are also presented. (Contains 149 references.) (Author)