Background: Attention deficit hyperactivity disorder (ADHD) and reading disability commonly co-occur because of shared genetic risk factors. However, the stability and change of these genetic influences and the predictive relationships underlying this association longitudinally remain unclear. Methods: ADHD symptoms and reading were assessed as continuous dimensions in a UK general population sample of approximately 7,000 twin pairs. Parent ratings of ADHD symptoms and teacher ratings of reading were obtained at two ages: middle childhood (ages 7-8 years) and early adolescence (ages 11-12 years). Cross-lagged quantitative genetic analyses were applied. Results: ADHD symptoms and reading significantly predicted each other over time. However, ADHD symptoms were a significantly stronger predictor of reading than vice versa. Inattentive and hyperactive-impulsive symptoms of ADHD both contributed to the prediction of reading, but inattentiveness was a significantly stronger predictor. Furthermore, ADHD symptoms and reading were highly heritable, and their association was primarily attributable to shared genetic influences. Despite notable genetic innovation for each trait, genetic factors involved in the association of ADHD symptoms and reading over time were highly stable. Conclusions: ADHD symptoms may put children at increased risk for reading problems and vice versa. Moreover, enduring genetic mechanisms appear to be important in the association of ADHD symptoms and reading over time.