New eating behavior disorders such as bigorexia (muscle dysmorphia) and orthorexia are appearing in developed countries. These disorders have not been officially recognized so that they are not classified as independent entities. The term orthorexia comes from the Greek word orthos (straight, proper) and orexia (appetite). It is characterized by the pathological obsession for biologically pure food, which leads to important dietary restrictions. Orthorexic patients exclude foods from their diets that they consider to be impure because they have herbicides, pesticides or artificial substances and they worry in excess about the techniques and materials used in the food elaboration. This obsession leads to loss of social relationships and affective dissatisfactions which, in turn, favors obsessive concern about food. In orthorexia, that patient initially wants to improve his/her health, treat a disease or lose weight. Finally, the diet becomes the most important part of their lives. We present a clinical case that responds to the characteristics of orthorexia. The differential diagnosis with chronic delusional disorder, anorexia nervosa and obsessive-compulsive disorder is carried out.