We amplified the 340 bp of mitochondrial DNA (mtDNA) by PCR including the recognized sequence of restriction enzyme of SfaN I. After amplification and digestion of SfaN I, two bands of 190 bp and 150 bp appeared in the mtDNA of four normal individuals but only one band of 340 bp appeared in the mtDNA with the mutation of G to A at the site of the nucleotide 11778 because such mutation destroyed the recognized sequence of SfaN I. We studied the mtDNAs of the patients with Leber's hereditary optic neuropathy from 19 Chinese families and their maternal relatives as well as the normal individuals i. e. the husbands of the female members of the pedigrees. The results show that 66.7% of the patients (30/45) and 54.7% of the maternal relatives (29/53) have such a mutation, while no such a mutation exists in the four normal individuals. So, we conclude that the mutation of mitochondrial DNA at position 11,778 is also a major cause of LHON in China.