A number of studies have recently proposed that input alone does not adequately promote both receptive and productive aspects of L2 vocabulary, and output is as necessary a condition as input for full development of L2 lexical proficiency. The goal of the present study is to address the question of what specific functions of output may affect L2 vocabulary development. This study considers that output could positively affect L2 vocabulary acquisition in terms of three aspects: a) noticing, b) retrieval, c) retention. The experiment was conducted with sixty-three learners of English in South Korea over the course of eight weeks. The results of ANOVA analyses of vocabulary and written production tests indicated that output plus input treatments affected more positively participants noticing, retrieval and retention of target vocabulary than the input only treatments did. The findings of this study suggest the importance of output in developing L2 vocabulary knowledge, as well as argue for the need to consider effective classroom instruction involving both input and output for receptive and productive vocabulary knowledge.