Using a serological test, different criteria have been established for classifying a case as primary or secondary dengue virus infection. Considering the dengue epidemiological situation in Cuba, IgG antibody response to dengue virus infection in serum samples from children and adults with a dengue 3 infection, in Havana city during the 2001-2002 epidemic was evaluated. Samples were collected on days 5-7 of fever onset and tested by an ELISA inhibition. A total of 713 serum samples positive for IgM antibody, 93 from children and 620 from adult patients were studied. Serum samples collected from healthy blood donors and patients not infected with dengue were included as controls. An IgG primary infection pattern was observed in sera collected from children, with titers of < or =20 in the 89.3% of the patients, while both, a primary and secondary patterns were observed in sera collected from adult patients with titers of < or =20 (13.4%) and > or =1280 (83.9%), respectively. These results permitted the definition of a primary or secondary case of dengue virus infection in serum samples collected during the acute phase of dengue virus infection.