This study aims to identify how mathematics teachers understand whole-class discussions and to know the influence of lesson study on the way they orchestrate these discussions. This is a qualitative study, conducted with two groups of middle school teachers. We analyze data concerning three teachers, Patrícia, Marta, and Diana, collected through initial individual interviews and observation of three research lessons. Data are analyzed by discourse analysis, establishing a relationship between the discourse about the teachers' understanding of whole-class discussions in the initial interview and their subsequent orchestration of this lesson moment. From this relation, we consider the influence of the lesson studies on the teachers' orchestration practice. The teachers' discourse in the interviews suggest the existence of a tension between their understanding of the goals of a whole-class discussion and their practice in orchestrating the discussion, causing several challenges. The lesson study contributed to lower these tensions and challenges through the definition of a fluid lesson structure and the appreciation, selection and recording of the students' activity, enriching the discussion. However, carrying out effective questioning and managing time proved to be complex challenges that deserve attention from further research.