&NA;Important advances were recently reported in mitochondrial diseases including Leber's optic neuropathy, Kearns‐Sayre syndrome, and myo‐neurogastrointestinal encephalopathy syndrome, combined hamartoma of the retina and retinal pigment epithelium, Hallermann‐Streiff syndrome, Norrie's disease, von Hippel‐Lindau disease, incontinentia pigmenti, and lattice corneal dystrophy type 2. Topics discussed include a general overview of mitochondrial DNA and mitochondrial‐derived inheritance, phenotypic mutational variations in patients with Leber's optic neuropathy, detection of platelet mitochondrial DNA deletions in patients with Kearns‐Sayre syndrome, and ophthalmic involvement in the myo‐neurogastrointestinal encephalopathy syndrome. The suspected association between combined hamartoma of the retina and retinal pigment epithelium and epiretinal membranes with neurofibromatosis type 2 is discussed, as is the similarity between Hallermann‐Streiff syndrome and oculodento‐digital dysplasia. Use of restriction fragment‐length polymorphisms to attain the prenatal diagnosis of Norrie's disease and the postnatal identification of at‐risk individuals in families in which von Hippel‐Lindau disease is segregating is presented, and an unusual case of incontinentia pigmenti in a man with Klinefelter's syndrome is described. Finally, a group of patients with lattice corneal dystrophy type 2 in whom a gelsolin gene mutation was identified is presented.Current Opinion in Ophthalmology 1993,4;VI:75‐82