We examined specific genetic hypotheses about the mode of transmission of Gilles de la Tourette's syndrome, by performing segregation analyses in 30 nuclear families identified through 27 index cases. Because data from earlier family studies had suggested that chronic tics and obsessive-compulsive disorder may be alternative phenotypic expressions of the diathesis of Tourette's syndrome, we used three diagnostic schemes to specify affected family members (Tourette's syndrome only; Tourette's syndrome or chronic tics; and Tourette's syndrome, chronic tics, or obsessive-compulsive disorder). The estimates of penetrance for the genotypes AA, Aa, and aa (A denotes the abnormal allele) in the analyses of subjects with Tourette's syndrome, chronic tics, or obsessive-compulsive disorder were 1.000, 1.000, and 0.002, respectively, for male subjects and 0.709, 0.709, and 0.000 for female subjects. These results predict that approximately 10 percent of all patients are phenocopies. We conclude that our analyses provide strong support for the hypothesis that obsessive-compulsive disorder is etiologically related to Tourette's syndrome and chronic tics in these families, and that Tourette's syndrome is inherited as a highly penetrant, sex-influenced, autosomal dominant trait.