Introduction and Aims. Despite cannabis use being very common in patients co-infected with HIV and hepatitis C virus (HCV), its effect on these patients' immune systems remains undocumented. Documenting the potential effect of cannabis use on HIV immunological markers would help caregivers make more targeted health recommendations to co-infected patients. We performed a longitudinal analysis of the relationship between cannabis use and peripheral blood CD4 T-cell measures in co-infected patients receiving antiretroviral therapy. Design and Methods. Cannabis use was assessed using annual self-administered questionnaires in 955 patients (2386 visits) enrolled in the ANRS CO13-HEPAVIH cohort. The effect of cannabis use on circulating CD4 T-cell count and percentage was estimated using multivariate linear regression models with generalised estimating equations. Sensitivity analyses were conducted after excluding visits where (i) tobacco use and (ii) smoking >=10 tobacco cigarettes/day were reported. Results. At the first visit, 48% of patients reported cannabis use during the previous four weeks, and 58% of these patients also smoked ≥10 tobacco cigarettes/day. After multiple adjustment, cannabis use was not significantly associated with either circulating CD4 T-cell count [model coefficient (95% confidence interval): 0.27 (À0.07; 0.62), P = 0.12] or percentage [À0.04 (À0.45; 0.36), P = 0.83]. Sensitivity analyses confirmed these results. Discussion and Conclusions. Findings show no evidence for a negative effect of cannabis use on circulating CD4 T-cell counts/percentages in HIV-HCV co-infected patients. In-depth immunological studies are needed to document whether cannabis has a harmful effect on CD4 levels in lungs and on cells' functional properties. [Marcellin F, Lions C, Rosenthal E, Roux P, Sogni P, Wittkop L, Protopopescu C, Spire B, Salmon-Ceron D, Dabis F, Carrieri MP, HEPAVIH ANRS CO13 Study Group. No significant effect of cannabis use on the count and percentage of circulating CD4 T-cells in HIV-HCV co-infected patients (ANRSCO13-HEPAVIH French cohort). Drug Alcohol Rev 2017;36:227-238]