The high endemicity of hepatitis B virus (HBV) infection and liver disease in Sardinia led us to assess the occurrence of HBV DNA in 1,411 sera of two selected groups of hepatitis B surface antigen (HBsAg)- negative blood donors: 793 with abnormal serum alanine aminotransferase (ALT) and 618 with normal serum ALT values (determined during routine testing of their blood donation). HBV DNA sequences were detected by dot-blot hybridization in 68 of 793 subjects (9%) with abnormal ALT but only in three of 618 subjects (0.5%) with normal ALT. HBV-core antibody (anti-HBc) was detected in 338 of 793 subjects (43%) with abnormal ALT as well as in 125 of 618 subjects (20.2%) with normal ALT. Among the 71 subjects positive for serum HBV DNA, 22 (31%) were positive for anti- HBc, while 49 (69%) were negative for all serologic markers of HBV infection. Thus, a high frequency of anti-HBc in apparently healthy HBsAg-negative individuals and a high prevalence of serum HBV DNA in the absence of immunologic markers of HBV infection suggest the existence of genetic variants of HBV that may be responsible for some of the presumed NANB hepatitis encountered in Sardinia and possibly other areas of high endemicity for HBV.