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In order to assess the genetic etiology of comorbid reading disability and attention deficit hyperactivity disorder (ADHD), reading performance and hyperactivity data (Diagnostic Interview for Children and Adolescents [DICA]; Herjanic, Campbell, & Reich, 1982) from 61 identical and 43 same-sex fraternal twin pairs tested in the Colorado Reading Project were subjected to multiple-regression analysis (DeFries & Fulker, 1985, 1988). Results suggest that approximately 45% of the proband deficit in reading is due to genetic factors that also influence hyperactivity. Moreover, heritable variation accounted for 70% of the observed covariance between the reading and hyperactivity measures. Thus, comorbid reading disability and ADHD (as measured by parental ratings on the DICA) is due, at least in part, to heritable influences.