Background: In the last 10-20 years, several new eating disorders have been discovered, and orthorexia nervosa (ON) is one of them. Aim: How prevalent is ON among 16-29 years old young fitness users in the fitness centres of Copenhagen. In addition, to investigate if the prevalence of ON is higher among women than men. Method: Descriptive, cross-sectional survey. The American questionnaire “Orthorexia Screen” (OS), translated into Danish, was used. The data collection took place at two fitness centres in Copenhagen among 16-29 year-olds. Results: The prevalence of signs on ON was 8.4 % (N=119), 14.1 % for women and 1.8 % for men. There was a significant difference between women and men (p=0.02). Conclusion: There is a need to develop a validated questionnaire which can help detect young people in risk of having problems related to ON.