Hepatitis delta virus (HDV) infection causes the most severe form of viral hepatitis. PEG-interferon alpha-2a (PEG-IFN alpha-2a) is the only effective treatment but its long-term clinical impact is unclear. The aim of this study was to investigate the long-term outcome after 48 weeks of pegylated interferon alpha-2a therapy. We performed a retrospective follow-up study of the Hep-Net-International-Delta-Hepatitis-Intervention-Study 1 (HIDIT-I trial). Patients had received 48 weeks of treatment with either PEG-IFN alpha-2a plus adefovir dipivoxil (ADV) (Group I), PEG-IFN alpha-2a alone (Group II) or adefovir dipivoxil alone (Group III). Liver-related complications were defined as liver-related death, liver transplantation, liver cancer and hepatic decompensation defined as development of Child-Pugh scores B or C or an increase in Model for End-stage Liver Disease (MELD) scores of five or more points in relation to baseline values. Patients were considered for further analysis when they were retreated with PEG-IFN alpha-2a. Follow-up data (at least 1 visit beyond post-treatment week 24) were available for 60 patients [Group I, (n = 19), Group II (n = 20), Group III (n = 21)]. Mean time of follow-up was 8.9 (1.6 - 13.4) years. 19 patients were retreated with IFN-based therapy: 42% (n = 8) in PEG-IFN alpha-2a arms and 58% (n = 11) in the adefovir only arm. Clinical complications on long-term follow-up occurred in 17 patients and were associated with nonresponse to therapy and baseline cirrhosis. The annual event-free survival rate in patients with cirrhosis vs noncirrhotic patients at year 5 and 10 was 70% vs 91% and 35% vs 76%. Long-term follow-up of a large randomized clinical trial suggests that off-treatment HDV RNA response to PEG-IFN alpha-2a treatment leads to improved clinical long-term outcome.