Background. Medically unexplained chronic fatigue in childhood may cause considerable disability and (by definition) its cause remains unclear. A study of fatigue in healthy twins has been undertaken to examine whether or not genetic factors play a part.

Method. A questionnaire survey of the main carers of an epidemiological population-based sample of 670 twin pairs who were asked about periods of unexplained and disabling fatigue in their twins. Out of 1340 individuals a period of disabling fatigue was reported for 92 (6·9%). Thirty-three (2·5%) reported disabling fatigue for more than 1 month. Zygosity could be confidently assigned in 98% of the sample providing 278 monozygotic (MZ) and 378 dizygotic (DZ) pairs. These data were analysed using a structural equation modelling approach.

Results. The results showed that disabling fatigue in childhood is highly familial with an MZ tetrachoric correlation (rMZ) of 0·81 and a DZ tetrachoric correlation (rDZ) of 0·59, for fatigue lasting at least a week. The most acceptable model using Akaike's information criteria, was one containing additive genetic effects (A) and shared environment (C) plus residual (or non-shared) environment (E). For fatigue lasting at least a month rMZ was 0·75 and rDZ 0·47. The most acceptable model included just A and E. However, the role of shared environment could not be conclusively rejected.

Conclusions. Unexplained disabling fatigue in childhood is substantially familial. Both genetic and shared environmental factors are worth further exploration in a search for the causes.