To investigate the incidence, determinants and significance of delayed clearance of serum HBsAg in chronic hepatitis B virus infection, a prospective follow-up study was conducted in two consecutive groups of patients. Group I consisted of 984 patients (859 men and 125 women) with biopsy-proven chronic type B hepatitis, whereas group II consisted of 1,598 asymptomatic chronic carriers (998 men and 600 women) with normal serum aminotransferase activity. During a mean follow-up period of 4.0 +/- 2.3 yr, 19 patients (1.9%) of group I cleared HBsAg from their serum, whereas 35 patients (2.2%) in group II did so in a mean follow-up period of 2.7 +/- 1.4 yr. The annual incidence of delayed serum HBsAg clearance was 0.5% in group I and 0.8% in group II (p less than 0.02). The cumulative probability of HBsAg clearance was also higher in group II than in group I (p less than 0.007). Antibodies to HBsAg developed in 9 patients (47.4%) with chronic hepatitis and in 11 (31.4%) asymptomatic carriers who cleared serum HBsAg. Those who were HBeAg negative and those older than 40 at entry and those who exhibited cirrhosis during follow-up had a higher incidence of delayed HBsAg clearance. Gender, initial histological changes and hepatitis delta virus infection did not influence the occurrence of HBsAg clearance. Serum HBV DNA was not detectable by slot-blot hybridization but was still detectable by polymerase chain reaction in serum specimens collected within 1 yr of HBsAg clearance. Liver biopsy performed later in 10 patients showed no significant hepatitis activity or tissue HBV DNA, HBsAg or HBcAg.(ABSTRACT TRUNCATED AT 250 WORDS)