As part of a pilot questionnaire survey of Gilles de la Tourette syndrome (TS), data on TS and tics for relatives of 75 patients with TS were collected. The frequencies of TS and/or tics among relatives were significantly heterogeneous in a familial pattern that suggests (1) that the disorder is transmitted, with tics alone being a milder manifestation, and that this severity difference is a threshold phenomenon related to transmission; (2) that the sex difference in prevalence is real because it was found among relatives of both male and female probands; and (3) that the sex difference is related to transmission as a threshold effect since female probands, although less common than male probands, had a higher proportion of affected relatives. These pilot data provide evidence for the transmission of TS and can be qualitatively explained by a genetic model of transmission.