Immunoglobulin M (IgM) antibody to dengue virus was examined from a total of 3,099 serum samples collected in southern Taiwan. Of 1,232 sera collected from a junior high school and four elementary schools in Liu-Chiu, 35 were IgM-positive, demonstrating that the dengue virus has been circulating on the island, despite the fact that no epidemic has been reported in the past 10 years. Sixteen of 925 sera collected from three elementary schools in Tung-Kang in 1991 were found to be IgM-positive and two of 192 sera from adults in the local community were positive. The IgM-positive subjects tended to be aggregated around a port. Fishing boats that had stopped in neighboring endemic countries were presumed to have introduced the virus periodically, causing a low level of inapparent infections. In the Kaohsiung area, two of 108 suspected clinical cases and four of 642 community-based sera were IgM-positive. Rapid urbanization has provided appropriate circumstances for vector breeding in this area and the high population density has also increased contact frequency between humans and mosquito vectors. This has, in turn, increased the possibility of silent transmission of the dengue virus via either intermittent reintroduction of the virus or continuation of inapparent infections or both. Establishment of a early warning system using the IgM antibody capture-enzyme-linked immunosorbent assay is suggested for effective monitoring of the disease.