The present study was undertaken to identify whether mitochondrial DNA (mtDNA) mutations were involved in the pathogenesis of Rett syndrome (RS). Mitochondrial DNA from 15 children with RS and 14 of their mothers was analyzed. No large deletions in mtDNA were found using Southern blot with a full-length mtDNA as a probe. Polymerase chain reaction amplification and single strand conformation polymorphism analysis showed mutations in region 2650-3000 encoding 16S rRNA of mtDNA in 13 cases of RS and 11 of their mothers. DNA sequence analysis and mismatch polymerase chain reaction results revealed a point mutation (C --> T) at position 2835 in 7 cases of RS and 6 of their mothers. The same mutation was not found in a total of 30 normal controls. These data indicate that mtDNA may play an important role in the pathogenesis of RS.