This study examined nation-wide data from China to assess client outcomes after 6-months of methadone treatment. Data on 19,026 clients enrolled between April 2008 and March 2010 were reviewed for changes in HIV-risk behaviours and emergence of new HIV cases. Multivariable logistic regression was used to identify factors associated with illicit drug use while in MMT. Clients reported reduced drug use and related risk behaviours and improved social functioning. There were 24 newly-identified cases of HIV. Continued drug use was associated with low attendance (OR = 5.98, 95% CI = 4.69-7.63), frequently seeing drug using friends (OR = 3.72 for daily vs. never, 95% CI = 3.18-4.34) and having a difficult family relationship (OR = 2.03 for difficult vs. good, 95% CI = 1.63-2.52). Methadone dose was not associated with continued drug use while in treatment. The Chinese MMT programme appears to be having a positive influence on those clients who remain in treatment, but needs to explore strategies to increase accessibility.