Task-based vocabulary learning has recently attracted lots of attention in the field of second language (L2) vocabulary acquisition. This article compares the effects of input and output tasks on English as a Foreign Language (EFL) learners' acquisition of vocabulary knowledge. Four intact classes of EFL learners were randomly assigned to one of four tasks of learning 18 target words through sentence reading exercises, i.e., matching, definition, choice, and combining. The definition task was found to be more effective than the other tasks in EFL vocabulary knowledge, irrespective of EFL proficiency. At each level of EFL proficiency, the matching and combining tasks performed equally well. The choice task had a small advantage over the matching and combining tasks at the learners' low EFL proficiency level, but not at the intermediate or high EFL proficiency level. Differential processing and division of attention may well affect task effectiveness.