We investigated the effects of social interaction and out-of-class study time on foreign language proficiency and use among two hundred students from a Japanese university who studied abroad for one month. Participants' use of English and English proficiency were measured before and after the programme. Interaction with people from other cultures and out-of-class study time while abroad were also reported. Based on social interaction and out-of-class study time, students were grouped into four clusters: an "Unengaged" cluster, with low levels of interaction and out-of-class study time; a "Balanced" cluster, with average levels of both interaction and study time; a "Studious" cluster, with high out-of-class study time but low interaction levels; and a "Communicative" cluster, with high interaction levels but low study time. Analyses of clusters' main effects revealed that overall, the "Studious" group had higher English proficiency, and the "Balanced" and "Communicative" groups reported higher use of English than the "Unengaged" group. While English proficiency increased to the same degree in all four groups, the "Balanced" group gained more in English use compared to the "Unengaged" group after study abroad. The results suggest the importance of how students spend their time abroad in promoting foreign language use.