Diagnostic criteria for Rett syndrome (RS) were developed by representatives of the International Rett Syndrome Association and the Centers for Disease Control for use in future clinical and epidemiological studies. Necessary criteria are: normal prenatal and perinatal period; normal psychomotor development through the first 6 months of life; normal head circumference at birth, with subsequent deceleration of head growth; loss of purposeful hand skills; severely impaired expressive and receptive language; apparent severe mental retardation; and gait apraxia and truncal apraxia/ataxia. Supportive criteria include breathing dysfunction, seizures, spasticity, scoliosis, and growth retardation. The diagnosis of RS is considered tentative until 2 to 5 years of age. The differential diagnosis includes other disorders associated with mental retardation, cerebral palsy, and seizure disorders. These diagnostic criteria for RS should foster reliable communication among investigators and enhance the epidemiological and clinical research of this important disorder.