English medium instruction (EMI) is a growing phenomenon in higher education contexts across the globe, and EMI programs are expanding rapidly in Japan as part of their internationalization efforts. This paper explores the relationship between student self-beliefs and their success in an EMI course within a bilingual business program. It analyzed direct measures of content course and preparatory course performances, proficiency, and questionnaire data from 139 students completing an English-mediated International Business course at a university in Japan. These data were supplemented by interviews with seven students. Multiple linear regression analysis found L2 proficiency, preparatory course performance, and self-efficacy to predict success in the EMI course. Our qualitative findings support these results, indicating that students with stronger efficacy put forth more effort and see course activities as development opportunities. These findings highlight the need for efficacy development opportunities for students entering EMI study.