We studied 648 hepatitis B surface antigen (HBsAg)- and/or anti-hepatitis C virus (HCV)-positive patients to evaluate the virologic and clinical characteristics of multiple hepatitis viral infection. We defined as Case B-C an HBsAg/anti-HCV positive patient and as Case b-C an anti-HCV/anti-HBc-positive, HBsAg/anti-HBs-negative patient. For each Case B-C we scheduled as Control-B an HBsAg positive and anti-HCV negative patient and as Control-C an HBsAg/anti-HBs/anti-hepatitis B core antigen (HBc)-negative and anti-HCV-positive patient. Control group C was used as the control also for Case group b-C. Serum HBV DNA by molecular hybridization was found more frequently in Control group B (54% of 161 patients) than in Case group B-C (35.7% of 84, P <.01). The prevalence of HBV wild type was similar in Case group B-C (14. 3%) and in Control group B (17.4%), whereas the e-minus strain was less frequent in Case group B-C (10.7% vs. 33%; P <.01). HBV DNA by polymerase chain reaction (PCR) was detected in 40.8% of 71 patients in Case group b-C. HCV RNA was detected more frequently in Control group C (90.7% of 130 patients) than in Case group B-C (65.2% of 69, P <.0001). Moderate or severe chronic hepatitis or cirrhosis were more frequent in Case group B-C (62.9% of 65 patients) than in Control group B (46.7% of 90, P <.05) or C (40.8% of 98, P <.005), and in Case group b-C (71.1% of 76) than in Control group C. Thus, in multiple hepatitis we observed a reciprocal inhibition of the viral genomes and a more severe liver disease. In Case group b-C, serum HBV DNA was frequent and the clinical presentation was severe.