Motivational interviewing (MI) is a directive, client-centered brief intervention to elicit behavior change by helping clients explore and resolve ambivalence. In this clinical trial, 152 outpatients and 56 inpatients entering public agencies for treatment of drug problems were randomly assigned to receive or not receive a single session of manual-guided MI. Drug use was assessed by self-report, urine toxicology, and collateral reports from significant others at baseline, 3, 6, 9, and 12 months. Contrary to prior reports, MI showed no effect on drug use outcomes when added to inpatient or outpatient treatment, although both groups showed substantial increases in abstinence from illicit drugs and alcohol.