Previous studies show that the symptoms of attention deficit hyperactivity disorder (ADHD) and lower intelligence quotient (IQ) covary in children. We investigated the aetiology of this association in a large population-based sample of 5-year-old twins. The twins were individually assessed on an IQ test, and data on ADHD symptoms were obtained from mother interviews and teacher ratings. Confirming previous studies, the phenotypic correlation between ADHD symptom scores and IQ was -0.3 and, in a categorical analysis, children with a Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) ADHD research diagnosis obtained IQ scores nine points lower, on average, than comparison children. We show here that the co-occurrence of ADHD and lower IQ has genetic origins: 86% of the association between ADHD symptom scores and IQ, and 100% of the association between ADHD diagnosis and IQ, was accounted for by genetic influences that are shared by ADHD and IQ. Some candidate genes for ADHD could also contribute to variation in IQ or vice versa.