To determine the frequency of obsessive, compulsive, and schizoid behaviors in Tourette syndrome (TS), we prospectively questioned 246 patients with TS, 17 with attention-deficit disorder (ADD), 15 with ADD due to a TS gene, and 47 random controls. The comparative frequency of obsessive, compulsive, and repetitive behaviors--such as obsessive unpleasant thoughts, obsessive silly thoughts, echolalia, palilalia, touching things excessively, touching things a specific number of times, touching others excessively, sexual touching, biting or hurting oneself, head banging, rocking, mimicking others, counting things, and occasional or frequent public exhibitionism--were significantly more common in TS patients than in controls. The frequency of each of these was much higher for grade 3 (severe) TS. Most of these behaviors also occurred significantly more often in individuals with ADD or in individuals with ADD secondary to TS (ADD 2(0) TS). When these features were combined into an obsessive-compulsive score, 45.4% of TS patients had a score of 4-15, whereas 8.5% of controls had a score of 4 or 5. These results indicate that obsessive-compulsive behaviors are an integral part of the expression of the TS gene and can be inherited as an autosomal dominant trait. Schizoid symptoms, such as thinking that people were watching them or plotting against them, were significantly more common in TS patients than in controls. Auditory hallucinations of hearing voices were present in 14.6% of TS patients, compared with 2.1% of controls (P = .02). These symptoms were absent in ADD patients but present in ADD 2(0) TS patients. These voices were often blamed for telling them to do bad things and were frequently identified with the devil. None of the controls had a total schizoid behavior score greater than 3, whereas 10.9% of the TS patients had scores of 4-10 (P = .02). This frequency increased to 20.6% in the grade 3 TS patients. These quantitative results confirm our clinical impression that some TS patients have paranoid ideations, often feel that people are out to get them, and hear voices.