The mitochondrial complex I genes were sequenced in seven Leber hereditary optic neuroretinopathy (LHON) families without the ND4/11,778 and ND1/3460 mutations. Four replacement mutations restricted only to LHON families were found, one in the ND1 gene at nt 4025, and three in the ND5 gene at nt 12,811, 13,637, and 13,967. The mutations did not change evolutionarily conserved amino acids suggesting that they are not primary LHON mutations in these families. They may be considered as secondary LHON mutations serving as exacerbating factors in an appropriate genetic background. A complex III mutation, cyt b/15,257, has been suggested to be one of the primary mutations causing LHON. Its presence was determined for 23 Finnish LHON families, and it was detected in two families harboring the ND4/11,778 mutation. Similarly, complex IV mutation COI/7444 was screened in Finnish LHON families, and it was found in one family carrying the ND1/3460 mutation.