This study investigated the effect of computer-assisted instruction (CAI) on adult second language (L2) learners' vocabulary recognition and production across high and low proficiency levels. Seventy-four participants were assigned to experimental (CAI) and control groups. All participants in the CAI group were categorized into high and low proficiency levels, based on their L2 vocabulary knowledge. The treatment lasted for one semester, 16 sessions, during which 16 passages were covered. While the CAI group individually worked on the passages uploaded on the CALL software, enhanced with hypertext annotations, the control group read the same passages through traditional teacher-directed instruction. The comparison of vocabulary recognition and production pretest and post-test scores revealed the significant effect of CAI on L2 learners' vocabulary uptake in the immediate and delayed post-tests. While no significant differences were found between the high and low proficiency participants in terms of their improvement from production pretest to the post-test, lower-level participants revealed more vocabulary recognition gains. The findings have pedagogical implications for L2 teachers, practitioners, and courseware designers in that they can rely on CALL software as a viable scaffolding tool for L2 vocabulary growth.