The study reported in this article investigated the comparative effects of two types of treatment--one of which emphasized input and the other output--on the vocabulary acquisition of young EFL learners. In the input-based instruction, the students were not required to produce output whereas in the production-based instruction the students were required to produce output. Thirty-six Japanese children aged 6-8 were divided into three groups (input-based, production-based and control group), received six weeks instruction and took four types of vocabulary tests as a pre-, post- and delayed post-test. The findings provide further evidence that both input-based and production-based instruction lead to both receptive and productive vocabulary knowledge. In general, the results show similar levels of effects for input-based and production-based instruction on vocabulary acquisition. However, an examination of process features indicates that the input-based tasks provided opportunities for richer interaction for the learners than the production-based activities. This may explain the better performance of the input-based group on the task-based comprehension test and the same levels of achievement in the production tests despite relatively fewer opportunities for second language (L2) production. (Contains 11 tables and 3 notes.)