The purpose of the DUCO research was to expand the evidence base for drug policy by quantifying the links between illegal drug use and crime. Data collection was conducted between December 2000 and June 2001. The inmate response rate was 73 percent. The majority of offenders reported using illegal drugs, and polydrug use was common. Regular illegal drug use during the 6 months prior to their most recent arrest, defined as current regular use, was reported by 62 percent of offenders. The drug-using careers for the sample almost always began with experimentation with cannabis, followed by amphetamines, heroin, and cocaine. Of those who reported drug use, 51 percent implicated alcohol or illegal drugs in all or most of their lifetime offending career, and 31 percent stated that alcohol or drugs had no impact on their criminal careers. Overall, addicted offenders were more likely to commit property and drug offenses regularly. Sixty-two percent of the total sample reported intoxication by illegal drugs or alcohol at the time of committing the most serious offense. By offender category, the proportions of offenders who attributed drugs or alcohol to their offending was greatest for fraud offenders, followed by regular multiple offenders and regular property offenders. Illegal drugs were most likely to be cited as a significant factor in the most serious offenses of these types of offenders. Intermittent offenders and homicide offenders were the offender groups least likely to have attributed their offending to illegal drugs or alcohol. When these types of offenders did attribute their offending to drugs or alcohol, alcohol predominated as having the greatest effect on the most serious offense. 6 figures, 4 tables, and 5 references