In this study the kinetics of humoral and cellular immune responses in first-time vaccinees and re-vaccinees with the yellow fever 17DD vaccine virus was analyzed. Flow cytometric analyses were used to determine percentual values of T and B cells in parallel to the yellow fever neutralizing antibody production. All lymphocyte subsets analyzed were augmented around the 30th post vaccination day, both for first-time vaccinees and re-vaccinees. CD3 + T cells increased from 30.8% (SE ± 4%) to 61.15% (SE ± 4.2%), CD4 + T cells from 22.4% (SE ± 3.6%) to 39.17% (SE ± 2%) with 43% of these cells corresponding to CD4 + CD45RO + T cells, CD8 + T cells from 15.2% (SE ± 2.9%) to 27% (SE ± 3%) with 70% corresponding to CD8 + CD45RO + T cells in first-time vaccinees. In revaccinees, the CD3 + T cells increased from 50.7% (SE ± 3%) to 80% (SE ± 2.3%), CD4 + T cells from 24.9% (SE ± 1.4%) to 40% (SE ± 3%) presenting a percentage of 95% CD4 + CD45RO + T cells, CD8 + T cells from 19.7% (SE ± 1.8%) to 25% (SE ± 2%Key words: yellow fever -vaccine -flow cytometry -neutralizing antibodies -lymphocyte subsets Yellow fever (YF) is a viral illness transmitted by infected mosquitoes (Aedes and Haemagogus genus) with yellow fever virus (YFV), which belongs to the Flavivirus genus, Flaviviridae family (Monath & Heinz 1996). It remains a serious health problem in endemic areas of tropical and subtropical Africa and South America (Vasconcelos 2003). Since the 19th century, studies about YF immunization have been accomplished (WHO 2001a). The YF 17D vaccine virus strain is one of the most effective and safe vaccines available. Immunization with the 17D vaccine strain induces a long-term neutralizing antibody response and provides excellent protection against infection with the virulent YFV (Wisseman & Sweet 1962). In 1937, the production of the YF 17DD vaccine virus substrain was started in Brazil, and has been in continuous use for 60 years (Post et al. 2001). The YFV 17DD vaccine is highly immunogenic and induces neutralizing antibody persistent at least 10 years and in some individuals up to 30 years or more (WHO 2001b).Although the YF 17D virus strain and the 17DD substrain are the most successful vaccines developed to date, recently, rare cases of postvaccinal neurological disorders have been recorded (Chan et al. 2001, Martin et al. 2001, Vasconcelos et al. 2001 .Despite the large literature on the humoral immune response to the YF vaccine virus, few works of the evaluation of cellular immune response, in particular T cell responses have been published (Reinhardt et al. 1998, Co et al. 2002, van der Most et al. 2002. The capacity of vaccines to activate the cellular immune system and induce T cell memory is an important mechanism of protection against wild-type viral infection. T cells often recognize more conserved epitopes that do not change due to antibody mediated selection pressure (Whitton & Oldstone 1996, Reinahrdt et al. 1998. Co and collaborators (2002) studying the T cell responses to YFV 17D in four volunteers could observ...