Orthorexia nervosa (ON) has in recent years attracted attention, but the scarcity of empirical research on the matter generates uncertainty about its progression. This study aims to gain insight into the developmental pathway of ON and factors contributing to it, to establish its etiology. In order to gain insights into health professionals' experience with patients that they identified as suffering from ON, this study adopted mixed methods, with a sequential exploratory design: first, semi-structured interviews (n = 10) were conducted; following, a questionnaire (n = 101) was administered online. Nutritionists, psychologists, psychotherapists, and support workers with experience in treating eating disorders in the Netherlands were the study's target group. Results shed light on environmental factors influencing the development of ON (e.g. pseudoscientific nutritional experts on social media), baseline risks (e.g. high level of education), initiating events (e.g. experiencing a break up), symptoms (e.g. feelings of depression and anxiety), diagnosis and type of treatment administered (e.g. cognitive-behavioral therapy). The current study is unique in its use of health professionals' experiential knowledge to assess how ON develops and who typically develops it. Furthermore, it contributes to the sparse literature on potential ways to treat ON. More top-down investigations into health professionals' experiences are encouraged as the literature on ON thickens.