Among 612 patients with chronic hepatitis type B, 24 patients tested positive for anti-C-100 antibody. Clinical features and viral markers of these 24 patients were studied. Anti-C-100 antibody was detected more often in patients negative for HBe antigen than those positive for this antigen (22/385 vs. 2/227 [P less than 0.01]). HCV RNA, as detected by reverse transcription and nested polymerase chain reaction, was positive in 20 of 22 patients negative for HBe antigen whereas it was detected in only one of two patients positive for HBe antigen. In a patient who seroconverted from HBe antigen to anti-HBe HCV RNA was undetectable in the HBe antigen positive phase and had become detectable after seroconversion. These data indicate that both hepatitis virus (type B and type C) can co-infect in a patient and that type C becomes active after the seroconversion from HBe antigen to anti-HBe.