Background and Aims: People who inject drugs (PWID) and other marginalized populations with high hepatitis C virus (HCV) infection rates represent a unique challenge for treatment initiation due to health, administrative and social barriers. We analysed the HCV cascade of care (CoC) in some vulnerable subpopulations in Madrid, Spain, when using a mobile point-of-care (PoC).Methods: From 2019 to 2021, a mobile unit was used to screen active HCV using a linkage-to-care and two-step PoC-based strategy. Viremic participants were grouped into four subgroups: PWID, homeless individuals and people with a mental health disorder (MHD) and alcohol use disorder (AUD). Logistic regression, and Cox and Aalen's additive models were used to analyse associated factors and differences between groups.Results: A prospectively recruited cohort of 214 HCV-infected individuals (73 PWID, 141 homeless, 57 with a MHD and 91 with AUD) participated in the study. The overall HCV CoC analysis found that 178 (83.1%) attended a hospital, 164 (76.6%) initiated direct-acting antiviral therapy and 141 (65.8%) completed therapy, of which 99 (95.2%) achieved sustained virological response (SVR). PWID were significantly less likely to initiate treatment, whereas individuals with AUD waited longer before starting the treatment. Both people with AUD and PWID were significantly less likely to complete HCV treatment.Conclusions: Overall, SVR was achieved in the majority of the participants treated.However, PWID need better linkage to care and treatment, whereas PWID and AUD need more support for treatment completion. K E Y W O R D S cascade of care, hepatitis C virus, marginalized populations, model of care, point-of-care test How to cite this article: Valencia J, Lazarus JV, Ceballos FC, et al. Differences in the hepatitis C virus cascade of care and time to initiation of therapy among vulnerable subpopulations using a mobile unit as point-of-care.