The purpose of the present study was to determine the rate of substance abuse in the juvenile detention house and to determine the relationship between crime and substance abuse and conduct disorder. Two hundred and thirty cases in the biggest juvenile detention house in Istanbul, Turkey were assessed according to Diagnostic and Statistical Manual of Mental Disorders (4th edn; DSM-IV) criteria. Law files and data of crime were examined. A total of 80 out of 230 juvenile detainees (34.8%) were found to have substance abuse excluding nicotine and alcohol. The substances abused in preferential order were cannabis (72.5%), volatile substances (21.3% bally and 3.7% thinner; 25%) and sedative hypnotic drugs and biperidents (2.5%). The rate of conduct disorder was 46.3% in substance abusers and 25.3% in the others (odds ratio: 2.536). The rate of substance abuse was 48.5% in the juveniles who had committed multiple crimes and 14.1% in the others (odds ratio: 5.735). The study shows that conduct disorder was very high in juvenile detainees. Conduct disorder was higher in substance-abusing than in non-abusing juvenile detainees. Substance-abusing juvenile detainees were found to have a higher detention rate than non-abusing juvenile detainees. There was a close relation between conduct disorder and substance abuse and multiple crimes. In the light of these results, diagnosis and treatment for conduct disorder in juvenile detainees are of great importance.