Abstract. Purpose: The aims of the present study were to investigate visual function, ocular motility and ocular characteristics in children and young adults with complex I deficiency. Material and Methods: In a prospective study with longitudinal follow-up, the visual and ocular outcome in 13 patients with deficiency in complex I [nicotine-amide adenine dinucleotide (NADH) dehydrogenase] in the mitochondrial respiratory chain is presented. The patients were diagnosed during 1995-2007 and assessed during 1997-2009 at a median age of 12.8 years (range 3.1-23.4). Results: Twelve of 13 patients had visual impairment and/or ocular pathology. Four of 10 patients who co-operated in visual assessment had a best corrected decimal visual acuity of A mutation in mitochondrial DNA. Only one patient had normal visual acuity and ocular outcome including refraction and visual fields. Follow-up time was median 3.0 years (range 0-11). Conclusion: Visual impairment, ocular motility problems and OA are common in children and young adults with complex I deficiency and should prompt the paediatric ophthalmologist to consider mitochondrial disorders.