In St. Crispin Hospital, Northampton, there are male identical twins (J.L. and P.L.) who for nearly a quarter of a century have suffered from an atypical psychosis. They were previously studied by Palmer (3). Their sister (B.L.) had a similar mental illness but she obtained relief from a leucotomy operation at an early stage. The mother was also affected, but the onset of illness in her case was later in life. There are two unaffected siblings.