In a study of the phenotypic characteristics of pedigrees of Leber's hereditary optic neuropathy positive for the mitochondrial DNA mutation at position 11778, 28 of 49 pedigrees were represented by singleton cases. Seven families, including six singleton pedigrees, had maternal family members with a mixture of mutant and normal mitochondrial DNA (heteroplasmy). Seventy-two affected individuals from 43 families showed a male predominance of 81.9% (59/72) and ages of onset of visual loss ranging from 8 to 60 years. The time interval between affected eyes averaged 1.8 months; the duration of progression of visual loss in each eye averaged 3.7 months. Visual acuity was 20/200 or worse in 107 of 109 (98.2%) eyes. Telangiectatic microangiopathy, disk pseudoedema, or vascular tortuosity, ophthalmoscopic features believed to be classic of Leber's hereditary optic neuropathy, were noted in 30 of 52 patients. Visual-evoked responses were typically absent or abnormal. Electrocardiograms, fluorescein angiograms, cerebrospinal fluid analyses, brain computed tomography, and magnetic resonance imaging were usually normal. There were no consistent neurologic or systemic illnesses associated with these Leber's pedigrees. In many cases, the diagnosis would not have been suspected because of the absence of a compatible family history, typical clinical profile, or ophthalmoscopic appearance. Genetic analysis showed the mitochondrial DNA mutation at position 11778, which established the diagnosis of Leber's hereditary optic neuropathy and has allowed for a broader view of the clinical features of this disease.