In a retrospective study from Ruud Vermeulen, the response of CFS patients to azithromycin, an antibiotic and immunomodulating drug, has been scored from the patients records and compared with clinical and laboratory data. Azithromycin was not the first choice therapy, but offered when the effect of counseling and L-carnitine was considered insufficient by the patient and the clinician. Lower plasma acetylcarnitine was observed in CFS patients who responded to azithromycin. The lower plasma acetylcarnitine level may indicate a decreased ability to counteract the action of the increased oxidative stress and/or an increased consumption of acetylcarnitine by the increased oxidative stress.