Twenty-eight patients from 25 maternal lineages with Leber's hereditary optic neuropathy (LHON) were investigated by restriction enzyme analysis for the presence or absence of the point mutation described by Wallace et al. The mutation was identified in 18 of 25 (72%) families with LHON. This provides further evidence that this mutation is present in the majority of patients with LHON. In 19 of these families with LHON, additional analysis using sequencing, oligonucleotide probing, and competitive oligonucleotide priming of PCR products was performed. In 14 cases with the site loss the point mutation was present, and five without the site loss had the wild type sequence in this region.