This study describes how medical students perceive professionalism and the context in which it is relevant to them. An understanding of how Phase 1 students perceive professionalism will help us to teach this subject more effectively. Phase 1 medical students are those in the first 2 years of a 5-year medical degree. Seventy-two undergraduate students from two UK medical schools participated in 13 semi-structured focus groups. Focus groups, carried out until thematic saturation occurred, were recorded and transcribed verbatim. Data were analysed and coded using NVivo 8, using a grounded theory approach with constant comparison. From the analysis, seven themes regarding professionalism emerged: the context of professionalism; role-modelling; scrutiny of behaviour; professional identity; 'switching on' professionalism; leniency (for students with regard to professional standards), and sacrifice (of freedom as an individual). Students regarded professionalism as being relevant in three contexts: the clinical, the university and the virtual. Students called for leniency during their undergraduate course, opposing the guidance from Good Medical Practice. Unique findings were the impact of clothing and the online social networking site Facebook on professional behaviour and identity. Changing clothing was described as a mechanism by which students 'switch on' their professional identity. Students perceived society to be struggling with the distinction between doctors as individuals and professionals. This extended to the students' online identities on Facebook. Institutions' expectations of high standards of professionalism were associated with a feeling of sacrifice by students caused by the perception of constantly 'being watched'; this perception was coupled with resentment of this intrusion. Students described the significant impact that role-modelling had on their professional attitudes. This research offers valuable insight into how Phase 1 medical students construct their personal and professional identities in both the offline and online environments. Acknowledging these learning mechanisms will enhance the development of a genuinely student-focused professionalism curriculum.