Abstract Objectives To examine the level of activity in online discussion forums for chronic fatigue syndrome/myalgic encephalopathy (CFS/ME) compared to other disorders. We hypothesized the level of activity to be higher in CFS/ME online discussion forums. Design Observational study Setting Norway, which has more than 80% household coverage in internet access, September 2009 Participants Twelve Norwegian disorder-related online discussion forums Main outcome measures Number of registered users and number of posted messages on each discussion forum Results Two forums were targeted towards individuals with CFS/ME. These forums had the highest number of registered users per estimated 1,000 cases in the population (50.5 per 1,000 and 29.7 per 1,000), followed by a site for drug dependency (5.4 per 1,000). Counting the number of posted messages per 1,000 cases gave similar indications of high online activity in the CFS/ME discussion forums. Conclusions CFS/ME online forums had more than ten times the relative activity of any other disorder or condition related forum. This high level of activity may have multiple explanations. Individuals suffering from a stigmatized condition of unknown aetiology may use the internet to look for explanations of symptoms or to seek out alternative treatments. Internet forum activity may also be reinforced by the creation of in-group identity and pre-morbid personality traits. More knowledge on the type and quality of information provided in online forums is urgently needed.