In the year 2001 a large dengue fever (DF)/dengue hemorrhagic fever (DHF) outbreak occurred in Nakhon Pathom Province, Thailand. Three thousand one hundred twelve cases of DHF were reported, an attack rate of 393 per 100,000 population. The Nakhon Pathom Provincial Health Office immediately carried out a control action according to WHO recommendations. Active serological surveys and viral RNA isolation were carried out to detect silent transmission of dengue virus in 329 healthy volunteers in Nakhon Pathom Province subdistricts where the dengue epidemic had the highest rate of infection of 2.5 per 1000. Eight point eight percent of these volunteers had a serum sample positive for DF/DHF virus IgM antibody. The highest prevalence occurred in the 15 to 40 year old group. In two instances viral RNA was detected by PCR and dengue serotype 3 was subsequently identified. The data support the hypothesis of subclinical infection with dengue virus. This high frequency of virus circulation combined with a high population density, urbanization and increasing breeding sites for mosquitoes, needs to be taken into account in the evaluation of viral transmission during and after epidemics. This underlines the importance of community-based control in informing people of their involvement in virus transmission and the importance of personal protection.