Eighteen female patients are described with the clinical features of Rett syndrome. Fifteen patients fulfill the criteria established by Hagberg et al hereas three represent clinical variants. Detailed biochemical and neurodiagnostic assessment was conducted in all patients. Reduction in cerebrospinal biogenic amine metabolites and characteristic alterations in respiratory, sleep, and EEG patterns were helpful in supporting the clinical diagnosis in the fifteen patients with typical features of Rett syndrome. In the remaining three patients, the modalities were much less helpful in establishing the diagnosis. Until a molecular marker is defined, diagnosis must depend on careful clinical assessment.