To examine the period between disappearance of hepatitis B e antigen (HBeAg) and appearance of anti-HBe, 48 patients with clinicopathologically verified chronic B hepatitis were followed every 1 to 3 months after HBeAg clearance. Sera were tested by radioimmunoassay for HBeAg and anti-HBe. Anti-HBe appeared in days to years, mostly (78.7%) within 1 year, after disappearance of HBeAg. Only 40.5% of patients had an “e-window” shorter than 1 month. Clinical and histological exacerbation preceding HBeAg clearance precipitated or accelerated appearance of anti-HBe. Since the patients in the “e-window” period were positive for DNA polymerase, it is suggested that the “e-window” reflects relative insensitivity of the radioimmunoassay rather than absence of HBeAg/anti-HBe. Therefore, HBeAg can reappear and clinical activity can relapse particularly during immunosuppressive therapy.