AIM: since patients with chronic fatigue syndrome (CFS) often present elevated levels of nitric oxide (NO) and low levels of physical activity, this study aimed at revealing possible correlations between NO concentration and physical activity. PATIENTS AND METHODS: thirty CFS patients and 29 age- and gender-matched sedentary controls wore an accelerometer for one week and underwent venous blood sampling at the beginning and the end of the week. RESULTS: CFS patients were significantly less active (p=0.001), but no significant differences in the amounts of NO (p=0.464 and 0.569) or interaction between NO levels and activity levels in either the CFS patients or controls were revealed. CONCLUSION: these results provide further evidence for reduced activity levels in CFS patients, but refute there being any interaction between the amount of blood NO and activity level in both groups. The blood NO was neither predictive of, nor dependent on the activity level in CFS.