Tourette's syndrome (TS) was studied in 18 pairs of monozygotic twins where at least one member of the twin set had TS. Sixteen twin sets were concordant for motor tics and had different ages of onset of motor tics (as reported by the mothers and/or medical records). Among these pairs, the earlier motor ticquer was more likely to have a more severe course of illness as assayed by two different indices. In the 10 sets concordant for vocal tics, the earlier vocal ticquer had a more severe course as assayed by only one index. The authors conclude that an early age of onset of motor tics may be the strongest predictor of a more severe life-time course of a tic disorder.Abstract Teaser