This article looks at the evidence for the heritability of Chronic Fatigue Syndrome or ME (CFS/ME). The role of genes and the environment and their potential interaction is discussed and the problems with studies in this area are explored. One of the main difficulties with studying CFS/ME is the fact that it is likely to be a heterogeneous condition. We need a clear definition of CFS/ME in children and sample sizes for genetic studies need to be much larger if we are to make an impact in this important area. In the meantime however, paediatricians shouldn't be surprised when they see a family where CFS/ME occurs in more than one person.