There have been relatively few studies of genetic factors in drug abuse. Childhood Attention Deficit Hyperactivity Disorder (ADHD) has been implicated as a risk factor, and pedigree studies of Tourette Syndrome (TS), a hereditary impulse disorder closely related to ADHD, show an increased prevalence of substance abuse in relatives. These observations suggest the genes for TS and ADHD may play an important role in the development of drug abuse. To examine this hypothesis 217 TS probands and 328 of their relatives, 58 ADHD probands and 35 of their relatives, and 50 controls were prospectively studied using a structured questionnaire based on the Diagnostic Interview Schedule. All subjects were Caucasians 16 to 49 years of age. The responses concerning the use of 8 different drugs and 8 different symptoms of drug abuse were compared. The results showed a highly significant increase in positive responses with increased loading for the TS and ADHD genes for 6 of the 8 drugs and all of the drug abuse symptoms. The percentage of positive responses in TS probands was markedly influenced by the presence of comorbid ADHD, as well as discipline, obsessive-compulsive, or alcohol problems. These results suggest that the genes responsible for TS and ADHD play an important role in drug abuse/dependence. The dopamine D2 receptor gene (DRD2) appears to be one of these genes since variants at this locus are significantly increased in frequency in TS, ADHD, conduct disorder and drug abuse.