A survey of 107 families of children with Rett syndrome was conducted to further define the behavioural phenotype. There was a high prevalence of behavioural and emotional problems, including episodes of anxiety (76 per cent), low mood (70 per cent) and self-injurious behaviour (49 per cent). Although there was no significant difference across the age-ranges studied, there was a tendency towards a reduction in these problems with increasing age, suggesting that the behavioural phenotype in Rett syndrome varies with age. Epilepsy occurred in over half the children, but there was no clear association with behavioural and emotional problems. A number of families had established strategies for coping with their child's anxiety and mood changes.