Spontaneous loss of HBsAg is infrequent in adult HBV carriers. Little is known about this serological change in children. In a prospective study of 420 hepatitis B virus–carrier children who were observed for 1 to 12 yr (mean = 4.3 yr), spontaneous loss of HBsAg occurred in 10 patients, with an average incidence of 0.6%/yr. The HBsAg clearance rate was significantly higher in children who had anti-HBe; children who were at an older age on entry; children whose mothers were HBsAg–; or children with severe liver histological changes detected while they were HBeAg+. Children who seroconverted from HBeAg to anti-HBe before the age of 6 or who had a peak serum ALT level above 100 IU/L were more likely to clear HBsAg. In all 10 patients who became HBsAg–, serum hepatitis B virus DNA became undetectable by both spot hybridization and the polymerase chain reaction, suggesting a complete clearance of the virus from serum. After the loss of HBsAg, the anti-HBs levels were higher in the children born to carrier mothers than in those born to noncarrier mothers. These findings suggest that chronic hepatitis B virus–carrier children rarely lose HBsAg, especially if they have been infected during the perinatal period and have mild histological changes. The poor humoral immune response to HBsAg may be a contributing factor in the establishment of carrier status during horizontal infection but may not be primarily involved in the establishment of carrier status during perinatal infection. (Hepatology 1992;15:382–386).