Trismus is a common problem after treatment of head and neck cancer. The Therabite is an effective treatment for trismus. To explore the factors that may influence Therabite exercise adherence, how these interrelate and to provide aims for interventions to increase adherence, the authors conducted a multi-centre, formal-evaluative qualitative retrospective study. 21 patients treated for head-neck cancer were interviewed in semi-structured, in-depth interviews. Internal motivation to exercise, the perceived effect, self-discipline and having a clear exercise goal influenced Therabite exercise adherence positively. Perceiving no effect, limitation in Therabite opening range and reaching the exercise goal or a plateau in mouth opening were negative influences. Pain, anxiety and the physiotherapist could influence adherence both positively and negatively. Based on the results, a model for Therabite exercise adherence was proposed. It is important to signal and assess the factors negatively influencing Therabite adherence, specifically before there is a perceived effect. Research is needed to examine why some patients do not achieve results despite high exercise adherence, to identify effective exercise regimens and to assess proposed interventions aimed to increase Therabite exercise adherence.