Three patients with Down's syndrome and coincident Tourette syndrome are presented. Two patients had trisomy for G-group chromosome and one patient had mosaic trisomy. All had characteristic features of Gilles de la Tourette syndrome with multiple motor and vocal tics. While Tourette syndrome has been reported in other chromosomopathies, this report appears to be the first to note an association with Down's syndrome. The authors hypothesize that neurotransmitter abnormalities existing in Down's syndrome may predispose such individuals to basal ganglia dysfunction and the subsequent development of tics. In particular, abnormalities in serotonin metabolism may be involved.