Readiness to change is a crucial issue in the treatment of substance use disorders. Experiences with methadone maintenance treatment (MMT) has shown that continuous drug and alcohol use with all its consequences characterize most MMT programs. In a prospective study of drug abusers seeking opiate agonist maintenance treatment in the City of Copenhagen, subjects were administered the Addiction Severity Index, and the Readiness Ruler for each of 11 different licit and illicit drugs by research technicians. Data was collected upon admission to the program and at a 18 month follow-up. Subjects who indicated they wanted to quit or cut down upon admission, reported less drug use at 18 month follow-up, after controlling for severity of drug problems at intake. Subjects who expressed readiness to change their drug use upon admission decreased their drug use. It is concluded that the Readiness Ruler measures a construct related to actual readiness, supporting its use in the clinical context.