During the period of January to December 2000, a total of 3,505 swine sera was collected from 208 farms, which are located throughout country, for the diagnosis of porcine reproductive and respiratory syndrome(PRRS). The antibody to porcine reproductive and respiratory syndrome virus(PRRS) was tested by indirect immunofluorescent antibody(IFA) test. Of 208 farms tested, at least one or more than one pigs was positive for PRRSV antibody in 188(90.4%) farms. The overall seroprevalence of PRRSV antibody was 45.1% (1581/3505). Most pigs were infected with PRRSV at around 50- to 60-day old. The seroprevalence of antibody varied with age. The highest seroprevalence of PRRSV antibody was observed in the growing pigs at around 80-day old. About one-thirds of adult pigs including boar, gilt and sow were positive to PRRSV antibody. In many farms, the infection of PRRSV was chronic and confined to grower and/or finisher. However, antibody was detected from all production phase in some farms.