To assess the prevalence of hepatitis delta virus (H delta V) in northern India, 204 adult patients with acute and chronic liver disease who were positive for hepatitis B virus (HBV) markers were screened for anti-H delta V antibody by enzyme-linked immunosorbent assay (ELISA). Anti-H delta V antibodies were positive in 29 (14.2%) patients. The incidence of H delta V infection was higher (21.4%) in patients with chronic liver disease when compared with those with acute viral hepatitis (10.7%) (P < 0.05). H delta V antibodies were positive in 16.6% of patients with fulminant hepatic failure (FHF) and in 25% of cases with hepatocellular carcinoma. Co-infections were significantly higher in acute hepatitis (80%), while superinfections predominated (66.7%) in chronic liver disease (P < 0.05). Our data show that H delta V is endemic in northern India and should be considered a major health problem.