We have studied the relationship between the cytokine production induced in vivo by prolonged isometric exercise and the symptom complex marked by fatigue in patients with chronic fatigue syndrome (CFS). Twelve male patients and 13 matched male control subjects undertook an isometric hand-grip exercise protocol utilizing dynamometers. Subjects undertook 30 minutes of exercise, for which the target force was set at 40% of the maximal voluntary contraction and the duty cycle was 50%. Prior to, during, and for 24 hours following the exercise, blood samples were collected and assayed for the presence of cytokines, including interferon-gamma and interferon-alpha, interleukin-1 beta, and tumor necrosis factor-alpha. At those times subjects also completed the Profile of Mood States (POMS) questionnaire, which served as a measure of changes in subjective fatigue. No significant alteration in the level of any of the cytokines in the plasma of patients or control subjects was detected before, during, or after exercise. Surprisingly, the patients' levels of fatigue, depression, and confusion, as measured by the POMS, decreased in response to the exercise. These data do not confirm the presence of an immunologic process correlating with the exacerbation of fatigue after exercise experienced by patients with CFS. Limitations in the study design and in the sensitivity of the cytokine assays may have affected our results.