Rett syndrome (RS) is a condition apparently limited to females characterized by normal early development followed by the abrupt loss of acquired function and beginning autistic behavior in late infancy. Manifestations in RS include decelerating head growth, unusual "hand-writing" movements, gait apraxia, neuromuscular tone disturbance, and seizures. No biochemical, hematologic, cytologic, or cytogenetic procedures have been shown to confirm the diagnosis of RS. With the exception of one family with 2 affected half-sisters, all cases have been sporadic. Six profoundly retarded residents of Central Wisconsin Center have manifestations of RS. Three are microcephalic and 3 have head circumferences at or below the 10th centile. All have seizures or abnormal EEG findings and all were ascertained on the basis of characteristic hand movements. No case of precocious puberty was observed. Two patients had severe scoliosis which was surgically treated. Detailed neuropathologic examination should be sought for every case of RS. The development of clearly defined minimal diagnostic criteria and the establishment of an international registry are desirable.