This paper reports a study of the nature and prevalence of behaviour problems in 258 children with mucopolysaccharide disorders. Questionnaire data obtained through the post was supplemented by home visits to 42 families in the sample and by regular discussions with families at meetings of the Society for Mucopolysaccharide Diseases. High rates of behaviour problems were found, particularly in children with Sanfilippo's and Hunter's disease aged 5 to 9 years. These included destructiveness, restlessness, and aggressiveness. Sleep problems were common across subtypes with an overall prevalence of 66%. Parents reported that they received little or no support in the management of these difficult behaviours. It is concluded that behaviour problems are a primary feature of the mucopolysaccharide disorders and place a major strain on families. Services to help families cope with these problems are urgently needed.