A model for the clinical delineation of atypical cases of Rett syndrome is presented. It is based on the presence, at age > or = 10 years, of combined clusters of at least 3 of 6 primary criteria and at least 5 of 11 supportive manifestations appearing through childhood with advancing age. The model was applied to 16 mentally retarded females, aged 11-47 years (median: 23) who were considered to manifest atypical variants of the syndrome (8 formes frustes, 6 late regression, 2 congenital variants). Two of the 16 patients had an early seizure history as the initial abnormality. In parallel, the number of supportive manifestations in a series of 41 females over 10 years of age with classic Rett syndrome are given. The differentiating power was tested on 8 patients with a chromosome-verified Angelman syndrome. It is concluded that the model applied here has the capacity to identify and distinguish Rett syndrome variants of different types, to sift out other developmental disorders in routine clinical work, and to have potential as a useful research tool.