This mixed-methods study investigated factors associated with fatigue, disability and school attendance in young people with severe CFS/ME. Participants’ illness experiences were also explored. Questionnaires were completed at baseline (T1) and approximately 5 months later (T2). There were 51 participants aged between 12 and 25, with a mean age of 18.8 years (SD 3.4). At T1, par- ticipants reported severe fatigue and poor social adjust- ment. Stronger fear avoidance beliefs at T1 were associated with higher fatigue at T2, and with worse social adjustment at T1 and T2. Female gender was associated with lower work/school attendance at T1 and T2 but not with higher fatigue or worse social adjustment. Having accessed treatment was associated with reporting lower levels of work/school attendance at T1 and T2. Multivariate analy- ses of key outcomes identified significant associations between stronger fear avoidance beliefs and worse social adjustment at T2, and between female gender and lower work/school attendance at T2. It was clear from the qual- itative data that severe CFS/ME negatively impacted on many aspects of young people’s lives. Fearful beliefs about activity could be targeted using cognitive-behavioural interventions.