Objectives: Reports of tiredness and poor quality sleep are common to both Chronic Fatigue Syndrome (CFS) and insomnia, despite evidence that sleep structure is not objectively impaired in these groups. Similar vulnerability and maintenance factors have been identified in both conditions, such as perfectionism, unhelpful beliefs and misattributions about symptoms. The aim of this study was to compare CFS and insomnia groups in terms of subjective sleep and fatigue symptoms and associated cognitive factors. Method: Using a cross-sectional design, CFS patients (n=18), community insomnia participants (n=18) and healthy community controls (n=19) were compared on a range of self-report questionnaires, including measures of psychological wellbeing (depression, anxiety, worry), insomnia, sleepiness and fatigue severity, cognitions about sleep and fatigue, and other cognitive variables associated with CFS and insomnia. Results: Between-group analyses identified that CFS and Insomnia participants did not differ significantly on the majority of variables. Compared to controls, both groups reported poorer psychological wellbeing and higher levels of insomnia, sleepiness, and sleep-related cognitions. Both groups also reported elevated perfectionism and unhelpful beliefs about emotions. Compared to the Insomnia group, CFS participants reported higher levels of fatigue, fatigue-related cognitions, and pre-sleep somatic experiences. Conclusions: This study found similarities between CFS and insomnia participants in terms of cognitive processes known to maintain insomnia. This indicates that it may be appropriate to use a transdiagnostic cognitive-behavioural approach to treating sleep disturbance in CFS. The results also indicate the importance of assessing for unhelpful fatigue-related beliefs and pre-sleep somatic complaints when working with the CFS population. Implications for further research are discussed.