Introduction: Hepatitis B is the single most important cause of viral hepatitis in developed and developing world. Hepatitis infection leads not only to acute viral hepatitis but in a number of cases it leads to carrier state which may progress to chronic liver disease, liver cirrohosis and ever liver carcinoma. Material and Method: Patients were taken attending medicine and blood bank of PMCH, Patna. Total no of cases were divided in to three groups. The diagnosis of acute viral hepatits was established on the basis of clinical presentation and liver function test characterised by hyperbilirubinaemia, serum transaminases etc. In the microbiology department serological test for hepatitis virus infection was done by ELISA method and result was noted. Result: In Group 1, Group 2 and Group 3 cases were tested . Observations showed that out of 38 cases of acute viral hepatis the most common aetiological agent was Hepatitis B virus 55.26%. Hepatitis B virus is most common in 3rd decade of age group. Males were more affected than female. Discussion: Tandon et al (1984) have reported an incidence of hepatitis B in 42% of sporadic cases of acute viral hepatitis. The presence of HbsAg in the serum of 10-25% of patient of chronic hepatitis has been demonstrated by Wright et al (1969), Gitinick et al. (1969), Mathews and Mackay (1970) and Boyes and Klatskin (1970). Thyagarjan et al. (1978) reported that out of 93 patients screened with feature of chronic active liver disease 17 were HbsAg (18.3%).Conclusion- Hepatitis B surface antigen is the most common marker in hepatitis B virus infection.