A stable, liver disease-free cohort of 369,725 was reconstructed as of 1984 for the study of hepatitis B virus seromarkers and liver diseases. The cohort consisted of male beneficiaries of the Korea Medical Insurance Corporation (KMIC) over 30 years of age and living nationwide. Subjects who were both negatives for HBsAg and anti-HBs (N = 274,037) were selected for incidence of hepatitis B. Data on test results of HBsAg and anti-HBs in 1984 and on hepatitis B occurrence during 1985-1986 were collected from the files of the KMIC. Linkage was done between these two data sets to measure the incidence rate through a longitudinal observation of the male population. Correction against misclassification error and duplicate claims was done by a sample survey and verification procedures. The incidence rate of hepatitis B was 17.13 per 100,000 person-years for acute viral hepatitis B and 15.74 for chronic hepatitis B, respectively. An increasing age-dependent pattern for acute hepatitis B was not so prominent in this population. However, the incidence rate of chronic hepatitis B steadily increased with age. The relative risk, estimated by a log-linear model for rate and constant hazard, was significantly higher in the over-60 age group than in the others. The incidence rate in the lower socioeconomic class was higher than in the others, although statistically not significant.