Chronic fatigue syndrome (CFS, also known as 'ME') is a debilitating disease which has a major impact on the lives of children and adults. Previous ALSPAC research studies have estimated how common CFS is (the 'prevalence' of CFS) among ALSPAC children at ages 13, 16 and 18 years. These studies also identified several important risk factors for CFS in children and young people, including family adversity and maternal mental health. ALSPAC data have not yet been used to investigate the prevalence of CFS in adults. ALSPAC mothers were asked about their medical history, including whether they had ever had CFS or ME, in questionnaires which were completed when ALSPAC children were age 8 and 11 years. Mothers' responses to these questions can be used to estimate the population prevalence of CFS in women in the UK. The relationship of CFS in ALSPAC children with CFS in ALSPAC mothers can also be investigated, and we can see whether risk factors for CFS in mothers are the same as the known risk factors for CFS in children.