Background: Previous large-scale studies have examined the effect of chronic hepatitis B virus (HBV) infection on overall and cause-specific mortality in individuals with HIV. However, few studies have collected data on the subclinical indicators of HBV that lead to these severe outcomes in the coinfected population. Objective: In this study, we aim to describe the procedures of a cohort study extension aimed at assessing HBV-DNA replication, serological markers of HBV (hepatitis B e antigen [HBeAg] and hepatitis B surface antigen), and liver fibrosis and how these subclinical outcomes relate to mortality in predominately tenofovir-treated, coinfected patients with HIV-HBV. We assessed the characteristics at cohort inclusion of those who participated in the cohort extension, as well as those who did not participate due to being lost to follow-up or death. Methods: Patients with HIV and chronic HBV who completed follow-up in a prospective cohort study conducted in 4 outpatient centers (Paris and Lyon, France; 2002-2011) were invited to participate in a cross-sectional visit from November 2016 to March 2018, during which a comprehensive evaluation of HIV-and HBV-related disease was undertaken. Virological and clinical data since the previous study visit were retrospectively collected. Results: Of the 308 individuals enrolled in the cohort, 147 (47.7%) participated in the cross-sectional study. At this visit, most participants were HBeAg negative (111/134, 82.8% with available data), had undetectable HBV DNA (124/132, 93.9% with available data), and were undergoing antiretroviral therapy containing tenofovir disoproxil fumarate or tenofovir alafenamide (114/147, 77.6%). There were no significant differences in characteristics at cohort inclusion between those who did and did not complete the cross-sectional visit, except for a lower proportion with an AIDS-defining illness (30/147, 20.5% vs 49/161, 30.4%, respectively; P=.04). Of the 161 nonparticipating individuals, 42 (26.1%) died, 41 (25.4%) were lost to follow-up and known to be alive, and 78 (48.4%) were lost to follow-up with unknown vital status. Most differences in characteristics at cohort inclusion were observed between deceased individuals and those participating in the cross-sectional visit or those lost to follow-up. With this extension, the median follow-up time of the overall cohort is presently 9.2 years (IQR 3.4-14.6). Conclusions: Extended follow-up of the French HIV-HBV cohort will provide important long-term data on the subclinical trajectory of HBV disease in the coinfected population. The biases due to the relatively high rate of those lost to follow-up need to be assessed in future studies of this cohort.