Heroin dependence remains a serious and costly public health problem, even in patients receiving methadone maintenance treatment. This study used a within-subject reversal design to assess the effectiveness of voucher-based abstinence reinforcement in reducing opiate use in patients receiving methadone maintenance treatment in an inner-city program. Throughout the study subjects received standard methadone maintenance treatment involving methadone, counseling, and urine monitoring (three times per week). Thirteen patients who continued to use opiates regularly during a 5-week baseline period were exposed to a 12-week program in which they received a voucher for each opiate-free urine sample provided: the vouchers had monetary values that increased as the number of consecutive opiate-free urines increased. Subjects continued receiving standard methadone maintenance for 8 weeks after discontinuation of the voucher program (return-to-baseline). Tukey's posthoc contrasts showed that the percentage of urine specimens that were positive for opiates decreased significantly when the voucher program was instituted. (P < or = 0.01) and then increased significantly when the voucher program was discontinued during the return-to-baseline condition (P < or = 0.01). Rates of opiate positive urines in the return-to-baseline condition remained significantly below the rates observed in the initial baseline period (P < or = 0.01). Overall, the study shows that voucher-based reinforcement contingencies can decrease opiate use in heroin dependent patients receiving methadone maintenance treatment.