BACKGROUND The study examined concurrent illicit heroin use among methadone maintenance clients in China and its association with clients' demographic characteristics, treatment experience, and personal social network. METHODS Face-to-face surveys were conducted with 178 clients randomly recruited from six methadone maintenance treatment (MMT) clinics in Sichuan, China. Concurrent heroin use was measured based on self-report of heroin use during the past 30 days and a confirmatory urine morphine test. The participants' demographic characteristics and treatment factors were measured and examined. The drug use status of their family members and friends was also assessed. RESULTS A total of 80 participants (44.9%) who either reported illicit heroin use in the past 30 days or had a positive urine test were defined as using heroin concurrently. Having drug-using friends was significantly associated with increased concurrent heroin use. Longer length of treatment (2 years or longer) was associated with increased concurrent heroin use. Among those who had both drug-using family members and friends, more women (71.4%) than men (50.0%) used heroin. For those who had no drug-using family members or friends, more men (34.8%) than women (20.8%) used heroin. CONCLUSIONS Study findings indicate an urgent need to address concurrent illicit heroin use among MMT clients. Further examination of the influence of social networks on concurrent drug abuse behavior is encouraged. Results also highlight the importance of understanding gender differences in treatment seeking and behavioral changes, which is crucial to the development of gender-specific treatment strategies.