Learning to read requires knowledge of word meanings for those words most commonly encountered in basic reading materials. Many young students lack the basic vocabulary knowledge needed to facilitate learning to read. Two randomized studies were conducted to test the effects of an online, computer-adaptive vocabulary instruction program designed to provide remedial instruction on word meanings for high frequency words. Study 1 was small in scope (N = 43) and tested whether the program could improve word knowledge on a corpus of 100 target words taught to all students in the treatment group. Study 2 was larger in scope (N = 192) and tested whether more extensive use of the computer-adaptive program, which teaches students individualized sets of words from a 4000 word corpus, could improve vocabulary test scores. Scaling up from 100 words in Study 1 to 4000 words in Study 2 necessarily corresponded to a proportionally equivalent decrease in posttest sensitivity to changes in students' vocabulary knowledge. It is argued that such a decrease in standardized test sensitivity requires post-intervention analyses to be conducted at the item-level rather than the posttest total score level. These studies suggest that computer-delivered vocabulary instruction may be an efficient mechanism for remediation of vocabulary deficits. Assessment of post-intervention results at the item-level may be appropriate in other attempts to scale up curricula from pilot studies to classroom use. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]