['In HIV-infected patients, data on immunogenicity of Yellow fever immunization are scarce, and there is conflicting evidence of the influence of CD4 T-cell count and plasma HIV RNA on neutralizing antibody titer (NT) after vaccine injection.', 'In this prospective cohort study, NT was measured in all consecutive HIV outpatients who had previously received at least 1 injection of Yellow fever vaccine. Risk factors for vaccine failure (NT < 1:10) and magnitude of NT according to dates of HIV diagnosis and immunization were assessed by logistic regression and general linear models.', 'Among 364 included patients, 24 (7%) had NT <1:10 after a mean delay of 8.4 years after immunization. Among patients immunized after HIV diagnosis (n = 240), NT <1:10 was associated only with detectable plasma HIV RNA at immunization. Among 79 patients with primary vaccination after diagnosis of HIV infection, higher HIV RNA at immunization was the unique independent risk factor for NT <1:10 [adjusted odds ratio (OR) = 3.73 per log10, 95% confidence interval (CI): 1.14 to 12.28]. Lower values of NT were independently associated with a shorter duration of undetectable plasma HIV RNA (OR = 1.05 per year, 95% CI: 1.005 to 1.09) and higher plasma HIV RNA (OR = 0.91 per log10, 95% CI: 0.84 to 0.99) at immunization.'] Is plasma HIV-RNA the key determinant of long-term antibody persistence after Yellow fever immunization in a cohort of 364 HIV-infected patients?