Influenza is an important public health problem in almost every winter of temperate countries, and is often associated with increased rates of hospitalization and death. In 1999, we started influenza surveillance with 4 voluntary sentinel physicians and the Public Health Center. From 1999 through 2003, 74 influenza viruses were isolated from 738 clinical specimens, which were collected from patients with Influenza like illness (ILI) in Seoul. The case definition for ILI was a fever over $38^{circ}C$ and systemic symptoms such as cough, or sore throat. ILI was the highest, 35.5% at the $20{sim}49$ age group and the rate of virus isolation was the highest, 20.6% at the $7{sim}19$ age group. Among isolated 74 influenza viruses, four of them were identified as A/H3N2 type in 1999-2000 season, seven of them were A/H3N2 in 2000-2001 season, fourteen were A/H1N1 in the 2001-2002 season and others were subtyped as influenza A/H3N2 viruses in the 2002-2003 season. Influenza viruses were collected 18.9% at Seocho-Gu, 18.8% at Dongjak-Gu and 7.6% at Kangbuk-Gu etc. and the isolate rate of virus had the area difference; Gwangjin-Gu 66%, Eunpyeong-Gu 65.6% Gangnam-Gu 20.0%, Kangbuk-Gu 19.6%. These findings may contribute towards the recommondation on the influenza vaccine formulation and development of influenza control measure based on the analyzed data.