BACKGROUND: Influenza is a pandemic disease because of the frequent antigenic variation. There is an international influenza network by WHO, but no national surveillance system has been established in Korea. The purpose of this study was to understand the influenza epidemiology by examining the incidence of influenza-like illness (ILI) and the subtype of isolated viruses in Korea during the winter of 1997 and 1998. METHODS: The consultation rates and clinical features of ILI were based on the weekly reports by 71 sentinel physicians (SP) from October 1997 to March 1998. Throat swabs from patients with ILI were collected and inoculated to Madin-Darby Canine Kidney (MDCK) cells. Isolates were identified by hemagglutination (HA), indirect immunofluorescence assay (IFA) and confirmed by hemagglutination inhibition (HI), and restriction fragment length polymorphism (RFLP). RESULTS: The incidence of ILI and virus isolation peaked in December 1997 and decreased to the baseline in February 1998. Influenza occurred mostly in 1- to 5-years old children and systemic symptoms were the main clinical features. One-hundred and thirty two influenza viruses among 2,071 specimens were isolated. The major subtype of isolates was A/H3N2 (A/Sydney/05/97). A/H1N1 subtype was isolated at low levels (4 strains), whereas B and A/ H5N1 were not. RFLP for M gene was compatible with those bands of A/H3N2 and A/H1N1. CONCLUSION: Influenza epidemiology was studied on the basis of a nationwide surveillance system during 1997 and 1998. The incidence of ILI and virus isolation peaked in December 1997 and predominant strains of isolates were closely related to A/H3N2. To establish a more effective system, it is necessary to increase the number of participating SP and laboratories.