This paper focuses on the role of pupil voice as a trigger for teacher learning and for improving teaching quality. This is investigated in the context of Lesson Study (LS), a professional development model that can incorporate pupil voice into teachers' collaborative reflections on lessons. Data are from two LS groups of mathematics teachers in London (one primary and one secondary school). Video-recorded pupil interviews and teacher discussions were transcribed. Episodes of teacher discussions were coded for reference to pupil input and subsequent impact on future plans. Qualitative analysis of discussions examined whether some pupils' input was favoured over others'. Results are significant in pointing to LS as a mechanism for attending to pupil voice. In so doing, it is suggested that pupil input provided a challenge for teachers in their interpretations of pupil learning, evaluating lessons and planning, and in contributing to teacher learning from LS.