This study examined the impact of methadone on drug-using and risky sexual behaviors among 557 heroin injection drug users in one of the first methadone programs in China. Structured questionnaires were administered to collect data on methadone utilization, drug use, and risky sexual behaviors. Blood samples were tested for blood-borne infections. Multivariate logistic analyses revealed that, among opioid-addicted individuals, methadone participants were less likely to use or inject drugs and share needles than nonparticipants. They also were less likely to have multiple sexual partners or have unprotected sex. Those currently enrolled in methadone maintenance had lower risk of using and injecting drugs than those who were no longer receiving methadone. Data suggest positive, short-term impact on these important behaviors, but further studies are needed to examine the long-term impact of methadone treatment on disease rates.