Purpose: The purpose of this study was to explore middle and secondary mathematics preservice teachers' (PST) perceptions of the benefits, challenges and impacts of implementing lesson study. Design/methodology/approach: This paper presents a single case study in the university context. Embedded units of analysis included a group of middle and a group of secondary mathematics PSTs, who were in the internship year of a teacher preparation program. Findings: This paper presents several perceived benefits, challenges and impacts according to PSTs. PSTs indicated benefits included their focused student observations and their collaboration in lesson design. Perceived challenges included observing as a nonparticipant observer and aspects of planning. The different impacts occurred in three phases: planning, observing during teaching and reflection. Research limitations/implications: First, the study participants included a small group of middle and secondary mathematics PSTs from one university in central Texas. Second, the PSTs in this study were not able to reteach the lesson, which may have influenced their perceptions. Practical implications: Mathematics teacher educators could use insights from this study as they implement lesson study or other field experiences to support PSTs in their growth as mathematics teachers. Originality/value: This paper examines PSTs' perceptions, which could benefit mathematics teacher educators as they consider how to introduce or implement lesson study with PSTs.