Studies elsewhere have suggested that immune dysfunction may be common in patients with chronic fatigue syndrome (CFS). The objective of this study was to assess the nature and extent of abnormalities in lymphocyte cell surface markers and NK cell activity in patients with CFS while controlling for genetic factors. A co-twin control study of immune system parameters was conducted for 22 pairs of monozygotic twins discordant for CFS and 9 healthy pairs of twins. The CFS twins had greater numbers of CD62L(+) T cells in several T cell subsets, although these differences did not achieve statistical significance. Significantly greater variability was noted in twins discordant for CFS than in the concordant healthy twins for 20 of 48 variables examined. The monozygotic co-twin control design is of unique value because of its ability to control for genetic influences on CFS; however, additional studies will be required to further assess immune dysregulation in this illness.