While numerous studies in mainstream education have drawn on the Theory of Planned Behaviour to investigate curriculum impact, in English Language Teaching contexts such research is scant. This study applies the theory to anticipate the impact of the Japanese national curriculum for English, by exploring the beliefs of senior high school teachers regarding the integration of grammar with communication-oriented teaching. The findings imply that for some teachers, adoption will be hindered, firstly, by misconceptions regarding high-stakes examinations, resulting in unfavourable attitudes and perceptions of social pressure to reject reform; and secondly, by insufficient resources, such as time and training. (Contains 5 tables and 1 figure.)