This is a presentation and discussion of clinical and laboratory data obtained on 13 girls with Rett syndrome, a progressive neurological disorder. The condition is thought to be far more prevalent than earlier reported. Family history in one patient showed presence of abnormal hand movements, increasing spasticity and psychomotor retardation in a paternal great grandaunt who died at 7 years. In the absence of chromosomal or biochemical markers, the characteristic disorder of hand movements can be used to distinguish this entity from other mental retardation, cerebral palsy and autism conditions. This report addresses the uniformity of clinical expression and highlights the differences between autism and Rett syndrome. Precocious puberty and respiratory alkalosis were not found in our patients. Feeding disorders were commonly present, and are often difficult to manage. The importance of diagnosis is emphasized as it influences long term management.