This paper investigates how Japanese lesson study has been modified in the context of the U.S., and how this modification has affected teacher agency based on the framework of the ecological approach proposed by Priestley et al. (2016). There are two major findings. First, Japanese lesson study has been adapted and modified in the U.S. as an effective form of professional development which tends to focus on a single subject, primarily mathematics. Math-focused lesson study has significantly contributed to the development of U.S. lesson study and promoted agency of math teachers, but may have hindered the expansion of lesson study into whole-school professional development, involving all subject teachers. Second, Japanese lesson study is inherently inseparable from the whole-school professional development called kounaikenshu, but, in the U.S., seems to have been adapted as a stand-alone activity. Consequently, the crucial role of long-term goals in lesson study has tended to be overlooked. The study concludes that lesson study disconnected from whole-school professional development may limit achievement of teacher agency, and in turn, professional identity.