AimTo estimate the prevalence of, and identify factors associated with, depression and anxiety among community-based methadone maintenance treatment (MMT) clients in China. DesignA cross-sectional survey. SettingNine MMT clinics, three each from three Chinese provinces (Yunnan, Anhui and Jiangsu) between October 2008 and February 2009. ParticipantsA total of 1301 MMT clients. MeasurementsA questionnaire, including the Zung Self-Rating Depression Scale (SDS) and Zung Self-Rating Anxiety Scale (SAS), and on-site urine drug testing. FindingsThe prevalence of depression (SDS score53) and anxiety (SAS score50) in our sample was 38.3% [95% confidence interval (CI)=35.7, 40.9] and 18.4% (95% CI=16.3, 20.5), respectively, with 14.2% (95% CI=12.3, 16.1) displaying symptoms of both. Sample prevalence rates for depression [mean=49.69, standard deviation (SD)=10.34] and anxiety (mean=40.98, SD=10.66) were higher than the national average for each (t((0.05/2, 1300))=19.2, P<0.001 and t((0.05/2, 1300))=8.0, P<0.001, respectively). Employing multi-level modelling techniques, gender (P=0.03) and employment status (P<0.001) were found to be associated significantly with depression in a single-level model; however, in a multi-level mixed model, only employment status (P<0.001) was associated with depression. Gender (P=0.03), education level (P=0.02), marital status (P=0.04), employment status (P<0.001), positive urine drug test results (P=0.02) and daily methadone dose (P<0.001) were found to be associated significantly with anxiety in a single-level model, while only employment status (P<0.01) and positive results for the urine drug test (P=0.04) were associated with anxiety in a multi-level mixed model. ConclusionsA considerable proportion of methadone maintenance treatment clients in China have experienced depression and anxiety during treatment. There is a need to provide tailored mental health interventions for this high-risk population.