This study aims to investigate the role of task-induced involvement and proficiency in the vocabulary development of adolescent Korean EFL learners. A total of 203 middle school students in Daejeon were assigned to 4 groups on the basis of the amount of task-induced involvement and the level of proficiency: Group 1 (low involvement + high proficiency), Group 2 (low involvement + low proficiency), Group 3 (high involvement + high proficiency) and Group 4 (high involvement + low proficiency). Immediate learning and delayed retention of 10 unfamiliar words were examined. Statistical analyses revealed that task-induced involvement load and proficiency played a significant role in both short-term vocabulary learning and retention. No interaction effect was observed between the two variables. These results suggest that a high level of task-induced involvement is important in the vocabulary development of middle school students at different proficiency levels. Pedagogical implications of the findings are discussed, and directions for further research are suggested.