Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder characterized by regression in cognition and adaptability with autistic behavior, stereotypical hand movements, epilepsy and ataxia. Over 120 different mutations in the methyl-CpG binding protein 2 gene (MECP2) have been reported in patients with RTT, but a genotype-phenotype correlation has not been established. We have studied MECP2 mutations in 142 Japanese sporadic patients diagnosed clinically as having RTT. Forty different mutations in MECP2 have been detected in 103 female patients. Common mutations were four missense mutations (T158M,P152R, R133C and R306C) observed in 34 cases and four nonsense mutations (R168X, R255X, R270X and R294X) detected in 38 cases. Among these, R133C, R306C, and R294X were associated with atypical RTT including the preserved speech variant type, T158M and R168X with typical clinical features of RTT, and P152R, R255X, and R270X with severe developmental delay. These results suggest a genotype-phenotype correlation RTT. However, a large scale study of adult RTT patients is required to determine more precisely the influence of MECP2 mutation types on the natural history and clinical phenotypes of RTT.