ContextAs unprofessional behaviour in physicians can compromise patient safety, all physicians should be willing and able to respond to lapses in professionalism. Although students endorse an obligation to respond to lapses, they experience difficulties in doing so. If medical educators knew how students respond and why they choose certain responses, they could support students in responding appropriately.ObjectivesThe aim of this study was to describe medical students' responses to professionalism lapses in peers and faculty staff, and to understand students' motivation for responding or not responding.MethodsWe conducted an explorative, qualitative study using template analysis, in which three researchers independently coded transcripts of semi‐structured, face‐to‐face interviews. We purposefully sampled 18 student representatives convening at a medical education conference. Preliminary open coding of a data subset yielded an initial template, which was applied to further data and modified as necessary. All transcripts were coded using the final template. Finally, three sensitising concepts from the Expectancy–Value–Cost model were used to map participants' responses.ResultsStudents mentioned having observed lapses in professionalism in both faculty staff and peers. Students' responses to these lapses were avoiding, addressing, reporting or initiating policy change. Generally, students were not motivated to respond if they did not know how to respond, if they believed responding was futile and if they feared retaliation. Students were motivated to respond if they were personally affected, if they perceived the individual as approachable and if they thought that the whole group of students could benefit from their actions. Expectancy of success, value and costs each appeared to be influenced by (inter)personal and system factors.ConclusionsThe Expectancy–Value–Cost model effectively explains students' motivation for responding to lapses. The (inter)personal and system factors influencing students' motivation to respond are modifiable and can be used by medical educators to enhance students' motivation to respond to lapses in professionalism observed in medical school.