The prevalence of Rett's syndrome was studied in a part of southwestern Sweden comprising five counties and the city of Gothenburg. In a population of 315469 children and adolescents, 6-17 years of age, 10 cases were detected, all girls. The corresponding prevalence was 0.65/10 000 girls, i.e. about twice that of phenylketonuria (PKU) in the same area. As progressive brain disorders/metabolic diseases together constitute 5-6% (1.5-2.0/10 000 children) of the aetiologies among severely mentally retarded persons of this age group in central Sweden, it can be concluded that within this group Rett's syndrome should be considered as an aetiological factor to think of in females. This syndrome might well be responsible for one-fourth to one-third of such cases among girls.