OBJECTIVE: The efficiency and efficacy of guided self-instruction for chronic fatigue syndrome (CFS) can be enhanced if it is known which patients will benefit from the intervention. This study aimed to identify moderators of treatment response. METHODS: This study is a secondary analysis of two randomized controlled trials evaluating the efficacy of guided self-instruction for CFS. A sample of 261 patients fulfilling US Center for Disease Control and Prevention criteria for CFS was randomly allocated to guided self-instruction or a wait list. The following potential treatment moderators were selected from the literature: age, fatigue severity, level of physical functioning, pain, level of depressive symptoms, self-efficacy with respect to fatigue, somatic attributions, avoidance of activity, and focus on bodily symptoms. Logistic and linear regression analyses were used with interaction term between treatment response and the potential moderator. RESULTS: Age, level of depression, and avoidance of activity moderated the response to guided self-instruction. Patients who were young, had low levels of depressive symptoms, and who had a low tendency to avoid activity benefited more from the intervention than older patients and patients with high levels of depressive symptoms and a strong tendency to avoid activity. CONCLUSION: Guided self-instruction is exclusively aimed at cognitions and behaviours that perpetuate fatigue. Patients with severe depressive symptom may need more specific interventions aimed at the reduction of depressive symptoms to profit from the intervention. Therefore we suggest that patients with substantial depressive symptoms be directly referred to regular cognitive behaviour therapy.