Vocabulary knowledge is of fundamental importance to reading comprehension, and many students lack the vocabulary knowledge necessary to facilitate learning to read. A study was conducted to determine the effects of an individualized, online vocabulary program on picture vocabulary test scores. Elementary summer school students (N = 43), entering grades 2-4, who scored poorly on a vocabulary pretest were randomly assigned to treatment or control conditions. Students in the treatment condition received computer-delivered vocabulary instruction on a stratified sample of 100 words selected from 4000 of the most common words in written English. Posttest scores on a picture vocabulary test showed that students in the treatment condition outperformed control students by more than one standard deviation. The computer-adaptive, individualized instruction provided by this vocabulary program addresses a need for efficiency in remediation of vocabulary deficits. Further study is planned to determine whether improved vocabulary performance mediated by this computer assisted language learning (CALL) program might transfer to broader measures of vocabulary knowledge or reading comprehension.