Leber's hereditary optic neuropathy (LHON) is a maternally inherited disease of the optic nerves associated with various mitochondrial DNA (mtDNA) mutations. Four of these mutations, at nucleotide positions (np) 3460, 11778, 14484 and 15257, have been postulated to be of primary pathogenetical importance. Previously, we described the molecular and clinical findings in patients with the 11778 and 14484 mutations. Here we describe the molecular and clinical findings of patients in eight pedigrees with the 3460 mutation and in three pedigrees with the 15 257 mutation. In all three 15257 positive pedigrees the 3460, the 11778 or the 14484 mutation was also found. The first combination has not been reported before. We compared the clinical findings in these pedigrees with those of the 3460, 11778 and 14484 positive pedigrees that lack the 15257 mutation. No significant differences were found with respect to the age of onset, visual outcome or the probability of developing LHON. We conclude that there is no evidence that the 15257 mutation, which has been reported in normal controls, has primary causal significance, because it may coincide with the 3460, 11778 and 14484 mutations. We presume that the 15257 mutation has no secondary pathogenic importance, since it has no clear contribution to the degree or the probability of phenotypic expression.