PURPOSE. To characterize the clinical features of childhoodonset Leber’s hereditary optic neuropathy (LHON) as defined by a pathogenic mtDNA mutation and age at onset equal to or less than 10 years of age. METHODS. Fifty-six LHON Italian pedigrees including 180 affected individuals were reviewed, and 14 of 18 patients with childhood LHON were enrolled. LHON was classified as acute bilateral, acute unilateral, slowly progressive, and subclinical, according to disease features. All patients underwent a complete ophthalmic examination and optical coherence tomography (OCT), including retinal nerve fiber layer (RNFL) and optic nerve head analysis (ONH), and were compared with age- and optic disc size–matched control groups. RESULTS. The prevalence of childhood LHON in this case series was 11.5%. Five patients had an acute bilateral course, three an acute unilateral course with subclinical signs in the fellow eye, and six a slowly progressive course. Four of five acute patients with acute bilateral disease experienced visual recovery. Slowly progressive cases presented a better visual acuity and visual field outcome than acute cases. A significant diffuse reduction of RNFL was evident in children with acute LHON compared with the control group, whereas a significant reduction of the temporal quadrant was present in the slowly progressive and subclinical LHON cases. Acute LHON children had a smaller disc area and vertical disc diameter than did the control subjects. CONCLUSIONS. This study systematically characterized for the first time the subgroup of LHON with childhood onset. The peculiar clinical and anatomic features of childhood LHON offer insights for the understanding of LHONs pathophysiology as well as a basis for the differential diagnosis of visual loss in childhood. (Invest Ophthalmol Vis Sci. 2006;47:5303–5309)