The prevalence, the epidemiology, the clinical and biochemical characteristics of hepatitis delta virus (HDV) infection were studied in patients with HBsAg-positive acute hepatitis, in those with chronic liver disease, and in apparently healthy carriers in Turkey. Fifty-eight of the 242 carriers of HBsAg (23.9%) and 31 of the 237 (13.1%) patients with acute HBsAg-positive hepatitis had serological evidence of HDV infection. Eleven of these individuals were HBsAg carriers with acute HDV superinfection. The prevalence of HDV infection was significantly (p less than 0.001) higher in patients with chronic liver disease (54/165; 32.7%) than in asymptomatic carriers of HBsAg (4/77; 5.2%). The highest prevalence (26/57; 45.6%) of HDV infection was found in patients at high risk of acquiring hepatitis virus infection (health care workers, hemodialysis patients, polytransfused patients) with chronic liver disease. Whereas the frequency of "severe" or fulminant hepatitis was similar in HBV infected patients (7.8%) and in HBV/HDV coinfected individuals (10%), the frequency of biphasic hepatitis was significantly (p less than 0.005) higher in the latter patients (30%) than in those with classical hepatitis B (7.8%). Chronic evolution of the disease was observed in 3.9% of the patients with classical hepatitis B and in 5% of those who had evidence of simultaneous HBV/HDV infection. The 10 carriers of HBsAg who survived the acute HDV superinfection developed chronic delta hepatitis. These findings indicate that HDV is endemic in Turkey and that its prevalence is highest among chronic HBsAg-positive hepatitis patients, implicating HDV as a major cause of liver disease among urban Turkis.