Five female patients have been presented with the typical clinical appearance of the Rett syndrome. A systematic evaluation of a variety of anatomic, physiologic and biochemical parameters has revealed two potential leads regarding the pathogenesis of this disorder. First, a reduction in the levels of neurotransmitter metabolites in cerebrospinal fluid was noted in each patient, a dopamine metabolite, homovanillic acid, being reduced to the greatest extent. Second, a sural nerve biopsy from one patient examined by electron microscopy revealed intact myelin with an axonopathy. Confirmation of these studies is necessary. This initial description of patients from the United States is quite consistent with previous reports by Professor Rett and Professor Hagberg and colleagues.