Abstract.? Jammes Y, Steinberg JG, Delliaux S (Aix-Marseille University, Marseille, France). Chronic fatigue syndrome: acute infection and history of physical activity affect resting levels and response to exercise of plasma oxidant/antioxidant status and heat shock proteins. J Intern Med 2012; doi: 10.1111/j.1365-2796.2011.02488.x. Objectives.? A history of high-level physical activity and/or acute infection might constitute stress factors affecting the plasma oxidant-antioxidant status and levels of heat shock proteins (HSPs) in patients with chronic fatigue syndrome (CFS). Design.? This case-control study compared data from 43 CFS patients to results from a matched control group of 23 healthy sedentary subjects. Setting and subjects.? Five patients had no relevant previous history (group I). Eighteen had practised high-level sport (group II), and severe acute infection had been diagnosed in nine patients (group III). A combination of sport practice and infection was noted in 11 patients (group IV). Interventions.? After examination at rest, all subjects performed a maximal cycling exercise test. Plasma levels of two markers of oxidative stress [thiobarbituric acid reactive substances (TBARS) and reduced ascorbic acid (RAA)] and both HSP27 and HSP70 were measured. Results.? At rest, compared with the control group, the TBARS level was higher in groups II, III and IV patients, and the RAA level was lower in groups III and IV. In addition, HSP70 levels were significantly lower in all CFS groups, compared with controls, but negative correlations were found between resting HSP27 and HSP70 levels and the history of physical activity. After exercise, the peak level of TBARS significantly increased in groups II, III and IV, and the variations in HSP27 and HSP70 were attenuated or suppressed, with the greatest effects in groups III and IV. Conclusion.? The presence of stress factors in the history of CFS patients is associated with severe oxidative stress and the suppression of protective HSP27 and HSP70 responses to exercise. © 2011 The Association for the Publication of the Journal of Internal Medicine.