Purpose – The purpose of this paper is to investigate the development of mathematics teachers’ pedagogical content knowledge (PCK) over successive cycles of lesson study. Utilising the framework of mathematical knowledge for teaching (MKT) (Ball et al., 2008), this research classifies features of PCK as utilised by post-primary mathematics teachers in their planning and reflection conversations in lesson study. The development of these features of PCK is then traced over successive cycles. Design/methodology/approach – In total, 12 teachers in two Irish post-primary schools participated in this research. Over the course of one academic year, these two groups of teachers completed a number of cycles of lesson study and qualitative data were generated through audio recordings of all lesson study meetings and through individual interviews with participants. Findings – Analysis of teacher dialogue reveals distinct features of knowledge of content and students (KCS) and knowledge of content and teaching (KCT) incorporated by these teachers in their planning and reflection conversations, providing empirical evidence of MKT in lesson study. The occurrence of these features of KCS and KCT in lesson study conversations increased over successive cycles, demonstrating teacher learning. Originality/value – This research contributes to the literature in expanding the theoretical underpinnings of teacher learning in lesson study. It also provides further empirical evidence of MKT (Ball et al., 2008) in teacher practice, specifically related to post-primary mathematics teachers.