Rett syndrome is a cause of severe learning disability in girls and is associated with a characteristic history and movement disorder. It is an X-linked dominant condition associated with mutations of the MECP2 gene on the distal part of the X-chromosome. If present in a male conceptus, the mutation is usually lethal. We present evidence to show that males can be affected by Rett syndrome. In the boy presented, this situation came about because cells containing the MECP2 mutation existed alongside a normal cell line. Somatic mosaicism could explain the occurrence of other X-linked dominant disorders in males, when they would normally be lethal.