Previous studies have suggested that obsessive-compulsive symptoms frequently occur among patients with Gilles de la Tourette's syndrome (TS). To examine the relationship between TS and obsessive-compulsive disorder (OCD), data from all first-degree relatives of TS probands were obtained with a semistructured interview designed to collect information on the presence of TS, other tic disorders, and neuropsychiatric illnesses during the lifetime of the individual. The rate of OCD among first-degree relatives was significantly increased over estimates from the general population and a control sample of adoptive relatives. The rates of TS, OCD, and chronic multiple tics (CMT) were virtually the same in families of probands with OCD (TS +/- OCD) when compared with families of probands without OCD (TS - OCD). Finally, the frequency of OCD without TS or CMT among first-degree relatives was significantly elevated in families of both TS + OCD and TS - OCD probands, suggesting that some forms of OCD may represent an alternative expression of the factors responsible for TS and/or CMT.