Abstract
The present study explores the outcomes of early bilingualism for speakers of the society's majority language in a Japanese context. English and Japanese proficiency of 48 Japanese high school students attending an English medium international school in Japan were assessed. Cummins' common underlying proficiency (CUP) theory was supported in both basic interpersonal communicative skills (BICS) and cognitive academic language proficiency (CALP). In addition, initial schooling in Japanese seemed to help students develop the highest level of balanced bilingualism found in the school. As a whole, for Japanese majority students attending an international school in Japan, systematic teaching of Japanese academic and technical skills in students' earlier years of schooling may enhance cognitive academic proficiency in their two languages.