In chronic fatigue syndrome (CFS), facilitating, initiating, and perpetuating factors are distinguished. Although somatic factors might have initiated symptoms in CFS, they do not explain the persistence of fatigue. Cognitive behavior therapy (CBT) for CFS focuses on factors that perpetuate and prolong symptoms. Recently it has been shown that, based on their level of activity, two groups of patients can be distinguished. For so-called "relatively active" CFS patients, the main perpetuating factors are nonaccepting and demanding cognitions leading to bursts of activity. For so-called "passive" CFS patients, their fear that activity might worsen their symptoms (which results in an avoidance of activity) is the most important perpetuating factor. These differences in perpetuating factors result in separate treatment manuals for relatively active and for passive CFS patients. Before describing the treatment manuals, we outline basic assumptions, considerations before starting CBT for CFS, and ways to determine the activity pattern. (Contains 2 figures and 2 tables.)