Reading disability (dyslexia) is a major social, educational, and mental health problem. Although estimates of prevalence vary, up to 10-15% of school-age children have severe reading deficits in spite of average intelligence and adequate educational opportunity. That reading disability may have a constitutional basis has long been recognized, and results of twin and family studies suggest that one or more of its forms may be heritable; however, definitive evidence for a genetic aetiology has not been reported. Establishing a heritable basis for reading disability could suggest possible causes, give improved risk estimates, facilitate early diagnosis, and provide validity tests for ostensible subtypes. In this report, we apply a recently developed multiple regression analysis to data collected from a sample of 64 pairs of identical twins and 55 pairs of fraternal twins, in which at least one member of the pairs is reading disabled, and present evidence for a significant genetic aetiology.