To the Editor .—I am engaged in a study of twin pairs, where at least one child suffers from early childhood autism. The main part of the survey consists in ascertaining twin pairs from the parent members of Societies for Autistic Children which are now formed in various parts of the world. This method of ascertainment avoids bias of various types, providing all the twins represented in the membership of the Society are located. However it would be of interest from the genetic viewpoint to study also twin pairs not collected in this way. I would therefore be extremely grateful if any doctor knowing of such a twin pair could let me have information on whether the twins are monozygotic or dizygotic, and whether one or both twins is affected. Diagnostic criteria could be discussed at a later stage for each notified pair. Would any doctor knowing of such a