The mechanism of development of chronicity after acute hepatitis B infection has not been elucidated fully. Following a single source outbreak of hepatitis B among 79 adult women, three patients (4%) became chronic carriers of hepatitis B virus (HBV). We compared features of the virus and antibody response of the latter three patients with those of 12 HBeAg-positive cases with resolving infection. The virus genotype was D, antigenic subtype ayw2. Base sequence analysis of S- and C-gene regions revealed no differences between the two groups. During the acute illness the three patients who developed chronicity had a remarkable transient reduction of HBsAg, HBeAg, and HBV DNA levels at 14-20 weeks after infection, the time of HBeAg seroconversion in the patients who cleared the infection. One HBeAg-specific monoclonal antibody (HBOT.95A) used as solid-phase antibody in a sandwich enzyme immunoassay detected an increased HBeAg signal in 2 of the 3 patients that developed chronicity and in 1 of the 12 patients who recovered. The latter patient had an exceptional long period of HBsAg antigenemia. Standard HBeAg assays detected HBeAg in all cases. HBeAg and anti-HBe-positive serum samples from the patients who recovered could inhibit the HBOT.95A response. The results suggest that chronic hepatitis B develops after an interruption of immune clearance. Differentiation of the antibody response to HBeAg may help to find patients with an increased risk for this interrupted immune clearance who might be candidates for an early intervention therapy.