This paper reports the outcomes of a "lesson study" project conducted in a mathematics department with four serving teachers in a secondary school in England. Using Dudley's lesson study framework and drawing on Hargreaves and Fullan's notion of professional capital, the feasibility and value of collaborative lesson study as a vehicle for the development of teacher learning were explored. Planning and evaluation meetings as well as end-of-project interviews were analysed to investigate how teachers planned research lessons together and how these were evaluated. Despite time constraints, teachers who engaged in lesson study reported that the process improved understanding of their students; that collaboration helped them to develop less-teacher-centred approaches and created a stronger sense of teacher community. The project demonstrated that lesson study has potential as an alternative or complementary model of teachers' learning, but it also throws up substantive organisational challenges if its use is to expand.