Obsessive-compulsive disorder (OCD) and obsessive-compulsive symptoms (OCS) exhibit a familial pattern of transmission. The different components of these conditions and the extent to which these components are inherited have not been studied well. A sample of 1,054 female twins, including both members of 527 pairs, from the Virginia Twin Registry returned questionnaires that included 20 items from the Padua Inventory of obsessive-compulsiveness. Their responses were used to estimate the heritability of the different factors of OCS in this population. Principal components analysis suggested two meaningful factors corresponding roughly to obsessions and compulsions. The best-fit model suggested heritabilities of 33 and 26%, respectively. The correlation between additive genetic effects on compulsiveness and obsessiveness was found to be +0.53. Self-report symptoms of obsessions and compulsions in women from the general population are moderately heritable and due, in part, to the same genetic risk factors. An understanding of the etiology of these symptoms is relevant to the study of OCD. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:791-796, 2000.