The heritable nature of reading disability has been well documented (DeFries & Alarcón, 1996), and possible abnormalities of brain structures have been associated with the disorder (Filipek, 1995). However, the etiology of individual differences in morphological brain measures has not been examined extensively. The purpose of this study was to apply behavioral genetic methods to assess the etiology of individual differences in neuroanatomical structures. Measures of reading performance, cognitive ability, and magnetic resonance imaging scans were obtained from 25 monozygotic (MZ) and 23 same-sex dizygotic (DZ) twin pairs with reading disability, and 9 MZ and 9 DZ control twin pairs participating in the Colorado Learning Disabilities Research Center. Results obtained from multiple regression analyses (DeFries & Fulker, 1985, 1988) of these twin data indicated that individual differences in the size of most cortical and subcortical structures were largely due to heritable influences. Moreover, estimates of heritability did not change appreciably after controlling for IQ and total brain size.