Both a prevalence study and a longitudinal study were conducted at several centers. In the prevalence study, active hepatitis delta viral infection was established by non-invasive serologic techniques such as hepatitis delta virus RNA/cDNA probes to detect hepatitis delta virus RNA and an immunoblotting method to detect hepatitis delta antigen. These tests avoided the need for liver biopsies to verify infection. In the longitudinal study, patients were assigned to a core or auxiliary groups with those in the core group sampled every six months for biochemical evidence of liver disease and those in the auxiliary group once a year. Serogroups 0,3,5, and 6 and other participants with evidence of delta hepatitis infection were assigned to the core group. Those patients who were immune to hepatitis B virus but were anti-hepatitis delta virus positive were assigned two controls, matched by center, age, sex, and hemophilia diagnosis and severity, from serogroups 5 who were without evidence of hepatitis delta virus infection. Thus, the role of delta virus infection in liver disease in hepatitis delta virus immune patients was evaluated. Follow-up continued for four years.