This study sought to determine the seroprevalence of the hepatitis D virus (HDV), the risk factors and its association with the severity of liver disease. Continuous patients at Tabriz and Tehran Hepatitis Clinics were enrolled during 2007-2008 in a cross-sectional study. Demographic data and possible risk factors for infection were recorded for all hepatitis B surface antigen positive patients. The blood samples of 847 patients infected with the hepatitis B virus were evaluated. The seroprevalence of HDV was 9.3%. This rate was significantly higher after reaching 40 years of age. The rate was 12.7% in patients with chronic hepatitis B and 4.7% in patients with in-active hepatitis B; the difference was statistically significant. A history of dental interventions and several trips abroad were good predictors of HDV infection in logistic regression. No significant difference in liver function tests was found. The seroprevalence of HDV was higher than in some other studies from Iran but a decrease was noted in younger age.