We examined levels of dengue virus type 3 (DENV-3) RNA in association with the type of infection (primary or secondary) in 42 patients with fatal and non-fatal outcomes in Rio de Janeiro, Brazil, 2002. Subjects with fatal outcomes had mean virus titers significantly higher than those who survived (12.5 vs. 7.9 log(10) RNA copies/ml). Because primary infections were confirmed among the fatal cases (52.1%), antibody-dependent enhancement alone did not explain all the cases of severe disease in this study population. These findings suggest that high levels of DENV-3 may have contributed to the severe form of dengue in Rio de Janeiro, 2002.