Lesson Study' is an approach to improving teaching, with very good effects on learning outcomes in Japan. This systematic process, as implemented in Hong Kong, draws on Ference Marton's theory of variation in teaching/learning, and involves teacher collaboration in a cycle of 'research lessons' to continuously refine one lesson on a selected topic. The approach is potentially empowering for all participants, not only teachers, and is more 'bottom-up' than most other processes of curriculum change. This paper is a report of one Lesson Study across one year, in the English department of one English-medium secondary school. The paper describes the successes and failures of this process, with video evidence where appropriate. The paper then evaluates learning outcomes at several participant levels. Specifically, student outcomes were positive to a large extent, with one surprising deficit; and teacher/teacher educator outcomes exhibited considerable variation, posited as involving personal, cognitive and affective factors. The paper finally evaluates the suitability of the lesson study approach to language teaching, raising issues concerning alternative models of pedagogical grammar, for example, which might facilitate this potentially powerful tool for change.