To assess conduct in Tourette syndrome (TS), 47 controls, 246 TS patients, 17 attention-deficit-disorder (ADD), and 15 ADD patients with minor tics or a family history of TS (ADD 2(0) TS) were compared for the following behaviors: running away from home, lying, stealing, starting fires, vandalism, being in trouble with the law, fighting, shouting at parents or peers, attacking others, lack of respect for adults, short temper, hurting animals, feeling full of hate, being unable to stop fighting, and problems with drugs and alcohol. With the exception of running away from home and being in trouble with the law, TS patients were significantly different from controls in all other behaviors. When the components were combined for a total conduct score, only one (2.1%) of the controls had a score greater than 13, and he had TS. By contrast, 35% of the TS patients had scores greater than 13 (P less than .0005). The correlation coefficient between the total conduct score and ADD score was .48. Although the presence of ADD was an important factor in determining conduct in TS, other factors such as depression and compulsive behavior also played a contributing role. There was little correlation between the total conduct score and the number of tics. It is estimated that among non-economically disadvantaged children, 10%-30% of conduct disorder may be due to the presence of a TS gene.