McArdle disease is caused by inherited deficit of human muscle glycogen phosphorylase with subsequent blockade in muscle glycogenolysis. Patients usually experience severe exercise intolerance and 'chronic' skeletal muscle damage. We determined circulating levels of 27 cytokines in a group of 31 adult McArdle patients (15 male 16 female; mean (+/-S.E.M.) age: 39+/-3 years) and 29 healthy sedentary controls (14 male, 15 female) before and after an acute exercise bout involving no muscle damage (cycling). Patients had an ongoing state of muscle breakdown even when following a sedentary lifestyle (serum creatine kinase activity at baseline of 2590+/-461 Ul(-1) vs. 97+/-5 Ul(-1) in controls). Under resting conditions, neutrophil count (+20%) and circulating levels of several cytokines were significantly higher (P<or=0.05) in patients than in controls: tumor necrosis factor (TNF-alpha), interleukin (IL)-1ra, IL-10, IL-12 and IL-17. The myokine IL-6 significantly increased with exercise (P<0.05) in both groups. Our results suggest that McArdle disease is associated with low-level systemic inflammation whereas appropriate exercise induces a similar response in McArdle patients and healthy controls, with a significant increase in the anti-inflammatory myokine IL-6. Our results support the rationale for prescribing carefully supervised exercise training in these patients.