Orthorexia nervosa (ON) describes a pathological obsession with healthy eating to avoid ill health. In the Swedish context, ON is also understood in terms of unhealthy exercise. Fitness gyms are popular health-promoting places, but exercise-related problems, disordered eating and ON-like behaviour are increasing. Personal trainers (PTs) play an important role in detecting unhealthy behaviours. The aim of the present study was to illuminate PTs’ understandings of healthy and unhealthy exercise and eating behaviours in relation to orthorexia nervosa in a fitness gym context. Five focus groups with 14 PTs were conducted. These were analysed using interpretative qualitative content analysis and Becker’s model “Kinds of Deviance.” In contrast to PTs’ health norms (practicing balanced behaviours and contributing to well-being), ON was expressed mainly in terms of exercise behaviour and as being excessive and in total control. The PTs maintain that extreme behaviours are legitimized by an aggressive exercise trend in society and that they fear to falsely accuse clients of being pathological. Certain sport contexts (bodybuilding, fitness competitions and elite sports) and specific groups (fitness professionals) contribute to complicating PTs’ negotiations due to a competition, performance and/or profession norm, making it difficult to determine whether or not to intervene. © 2017 The Author(s)