Background McArdle disease is an autosomal recessive genetic disorder caused by a deficiency of the glycogen phosphorylase (myophosphorylase) enzyme, which muscles need to break down glycogen into glucose for energy. Symptoms include exercise intolerance, with fatigue, muscle pain, and cramps being manifested during the first few minutes of exercise, which may be accompanied by rhabdomyolysis. Case presentation This case report describes for the first time the clinical features, diagnosis and management of a 20 year-old patient with anorexia nervosa and McArdle disease, documented by means of muscle biopsy. Conclusion Anorexia nervosa and McArdle disease interact in a detrimental bidirectional way. In addition, some laboratory parameter alterations (e.g., elevated values of creatine kinase) commonly attributed to the specific features of eating disorders (e.g., excessive exercising) may delay the diagnosis of metabolic muscle diseases. On the other hand, the coexistence of a chronic disease, such as McArdle disease, whose management requires the adoption of a healthy lifestyle, can help to engage patients in actively addressing their eating disorder.