In theory, computer game-based learning can support several vocabulary learning affordances that have been identified in the foreign language learning research. In the observable evidence, learning with computer games has been shown to improve performance on vocabulary recall tests. However, while simple recall can be a sign of learning, observation of skill application in communication is a better indicator of skill mastery. Further, observing this use in separate communicative contexts could constitute evidence of transferability of skills. Hence, this paper presents the results of two investigations of learning outcomes in EFL classes at a Japanese university using computer game-based lessons. The first study was a quasi-experiment comparing use of targeted words in a writing task between a group of students who participated in a computer game-based lesson, and a group of students who did not. The second study was a cross sectional analysis comparing use of targeted vocabulary in a writing task with amount of participation in computer game-based lessons. The results suggest that computer game-based approaches to foreign language education in real-world classrooms can improve transferability of learned vocabulary.