Fifty-two patients with Gilles de la Tourette syndrome (TS) diagnosed according to DSM-lll criteria and/or their parents were interviewed regarding symptoms among family members. Occurrences of TS and multiple tics (MT) among first-degree relatives were distinguished to determine the familial patterns of the two diagnoses. Comparing these data with family data collected on a random national sample of patients with TS, we found no differences between the two samples. Each separately and both combined showed that (1) MT seems to be a mild form of TS; (2) both MT and TS are transmitted in the same families; (3) the sex difference is real and not an artifact of ascertainment; and (4) the sex difference is related to the transmitted susceptibility as a threshold effect. A specific genetic mechanism has not been identified.