To determine the influence of family history on clinical expression of Tourette's syndrome (TS). Recent studies have suggested that clinical expression of TS is similar among sporadic (SP) and familial patients but may be influenced by bilineal (BIL) transmission of tics or obsessive-compulsive behavior (OCB) in high-density pedigrees. The authors used family history methodology, supported by direct examination of affected relatives in 73% of familial patients, to determine the frequency of SP TS, and of unilineal (UNL) and BIL transmission of tics or OCB in 111 consecutively ascertained juvenile TS patients. For individuals in each group, severity of tics, attention deficit hyperactivity disorder (ADHD), and OCB were assessed at presentation and after a mean follow-up interval of 2.6 years, using the Tourette's Syndrome Global Scale and the Clinical Global Impression scales. The phenomenology of OCB was evaluated using the symptom checklist of the Children's Yale-Brown Obsessive Compulsive Scale. The authors documented BIL transmission of tics in seven patients (6%). Patient age and sex were similar for the SP (n = 21; 19%), UNL (n = 66; 59%), and BIL (n = 24; 22%) groups, as was ADHD and tic severity at presentation and follow-up. Severity of OCB differed significantly between groups, with moderate to severe OCB affecting 5% of SP, 12% of UNL, and 37% of BIL patients at presentation (p = 0.007), and 5% of SP, 17% of UNL, and 54% of BIL patients at follow-up (p = 0.0001). Relative to UNL or SP patients, BIL patients were more likely to exhibit self-injurious behaviors (p = 0.0005). OCB is less prominent in SP than in familial TS, perhaps reflecting a more restricted pathophysiology in this subgroup. Although BIL transmission of tics is relatively infrequent in consecutive TS pedigrees, cotransmission of OCB from an otherwise unaffected parent is common and significantly influences development of OCB and self-injurious behaviors, but not tics, in offspring. Genetic heterogeneity, epigenetic factors, and gene-environment interactions may play a more important role than genetic dosage effects in determining tic severity in TS.