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, participants reported severe fatigue and poor social adjustment. 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 analyses 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 qualitative 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.