OBJECTIVE: Previous literature has identified the importance of interpersonal processes for patient outcomes in chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME), particularly in the context of significant other relationships. The current study investigated expressed emotion (EE), examining the independent effects of critical comments and emotional overinvolvement (EOI) in association with patient outcomes. METHOD: Fifty-five patients with CFS/ME and their significant others were recruited from specialist CFS/ME services. Significant other EE status was coded from a modified Camberwell Family Interview. Patient outcomes (fatigue severity, disability, and depression) were derived from questionnaire measures. Forty-four patients (80%) completed follow-up questionnaires 6-months after recruitment. RESULTS: Significant other high-EE categorized by both high levels of critical comments and high EOI was predictive of worse fatigue severity at follow-up. High-critical EE was associated with higher levels of patient depressive symptoms longitudinally; depressive symptoms were observed to mediate the relationship between high critical comments and fatigue severity reported at follow-up. There were higher rates of high-EE in parents than in partners, and this was because of higher rates of EOI in parents. CONCLUSIONS: Patients with high-EE significant others demonstrated poorer outcomes at follow-up compared with patients in low-EE dyads. One mechanism for this appears to be as a result of increased patient depression. Future research should seek to further clarify whether the role of interpersonal processes in CFS/ME differs across different patient-significant other relationships. The development of significant other-focused treatment interventions may be particularly beneficial for both patients and significant others. (PsycINFO Database Record (c) 2014 APA, all rights reserved).