To elucidate the epidemic pattern of a dengue outbreak in southern Taiwan during 1987–8, antibody prevalence rates were investigated in paired sera collected in both epidemic (Kaohsiung) and non-epidemic (Tainan) areas. In Kaohsiung, the IgG prevalence rate in 1989 was significantly higher (9·23%) than that in 1988 (5·29%) suggesting that new infections continuously appeared after the first bleeding in 1988. Although IgG antibody persisted in most infected blood samples, waning of antibody occurred in 6/355 (1·69%) of Kaohsiung sera. IgM antibody was only detected in Kaohsiung sera, suggesting that Tainan was not involved in the outbreak. Because IgG antibody was present in some samples collected in 1989, but not in 1988, from the non-epidemic area, sporadic infections perhaps occurred. Additionally, 4/355 (1·13%) of Kaohsiung sera showed IgM antibody positive in both 1988 and 1989. In turn, secondary infections may have occurred because of circulation of multiple-types of the virus. The possible relationship between low levels of dengue haemorrhagic fever (DHF) and the loss of IgG antibodies over time is also discussed.