Objectives : This study was performed to determine the incidence density and the prevalence of sero-positive hepatitis C from 1999 to 2002 among adults aged 20 and over residing in Seoul and the Gyeonggi province. Method : The data for period was obtained from 114,635 adults, residing in Seoul or the Gyeonggi province, who had undertaken comprehensive health screening tests from Jan 1999 to Dec 2002 in a University hospital in Seoul. Among them, subjects with sero-negative status against hepatitis C were selected (21,408 in 1999, 28,830 in 2000) and then followed up until Dec 2002 to determine the incidence of hepatitis C during this period. The serum was tested with the immunoradiometric assay (IRMA) which uses third generation HCV antibody. Age adjusted rates were estimated by direct standardization using a reference population of 2000 aged from 20 to 80 years. Results : The prevalence of anti-HCV from 1999 to 2002 was 2.1 per 1000 persons(95% CI $1.8{sim}2.4$). Male showed 1.7 per 1000 persons (95% CI $1.4{sim}2.1$), while female showed 2.7 per 1000 persons(95% CI $2.2{sim}3.2$). Age?sex adjusted rate showed 2.8 per 1000 persons (95% CI $2.64{sim}2.96$), which is lower than the results of some previous study. The prevalence showed a significantly increasing pattern with age both in males and females (p<0.05). The incidence density of anti-HCV among the population aged 20 and over was 1.1 per 104 person-years at risk (95% CI $0.6{sim}2.4$); 1.2 (95% CI $0.6{sim}2.7$) for males and 0.8 (95% CI $0.6{sim}4.2$) for females. Age adjusted incidence density was 2.91 per 104 person-years at risk (95% CI $2.43{sim}3.38$) for those aged 20 and over. It showed an increasing pattern with age (p<0.05), especially for those age over 50 years. Conclusion : The study subjects for this study were supposedly healthier than the general population so the prevalence and incidence for the general population are thought to be higher than the results of the present study.