The author reports eight cases of the Rett syndrome, or dementia-ataxia-autism, in girls. The cases satisfy the following criteria: Normal development in the first mos of life. Profound deterioration of the mental status over a period of several mos. Behavioral pseudoautistic abnormalities. Presence of neurological signs such as ataxia, myoclonus and hyperreflexia. Normal head circumference at birth, but subsequent subnormal growth. EEG abnormalities. Slow progression of the disease after the period of rapid deterioration. The constellation of the signs and symptoms, and the occurrence only in girls, make this clinical picture quite distinct. There is no definitely known biochemical or chromosomal abnormality. The psychological profile, though homogeneous in all the patients, is not pathognomonic, and a very similar behavioral pattern can be observed in other organic brain syndromes.