Hepatitis D virus (HDV) is a defective RNA virus dependent on hepatitis B virus (HBV) infection for its replication and expression. It is known that coexistent infection with HDV tends to aggravate the course of HBV-associated liver disease. This study was carried out to determine the seroprevalence of HDV among hepatitis B surface antigen (HBsAg)-positive individuals in the northeast part of Iran. 139 HBsAg-positive subjects detected from a population-based single stage cluster sampling in Golestan province of Iran were enrolled. All cases were evaluated for the presence of anti-HDV antibodies using commercially available enzyme-linked immunoabsorbent assay kits. Logistic regression was used to determine the relationship between independent variables and HDV seropositivity. Of 139 cases, 68 were males (48.9%) and 71 were females (51.1%). The mean age was 41.9 +/- 11.3 years (range, 25-64 years). Anti-HDV antibody was positive in 8 subjects (5.8%), with a female predominance (9.9% vs 1.5%, p=0.06; odds ratio, 7.32; 95% confidence interval, 0.87-61.23). No significant relationship was seen between anti-HDV seropositivity and demographic factors such as age, place of residence and marital status. These findings show that HDV infection is endemic in Golestan province (northeast) of Iran. Seroprevalence of anti-HDV in the present study was higher than in some previous studies from other parts of Iran. Our results suggest that the prevalence of HBV/HDV coinfection in Golestan province of Iran has increased during the last decade. Therefore, practitioners and health care managers should be made aware of the risk of dual infection with HBV and HDV.