Much evidence supports the hypothesis that Australia antigen (Au) causes viral hepatitis. We found Au(1) in 30 of 43 drug abusers with hepatitis (69.8%), a similar frequency to posttransfusion hepatitis (56.3%), but different from infectious hepatitis (30.1%). There were no distinctive histologic features in liver biopsies from drug abusers. Two of nine drug abusers with Au(1) followed serially, developed persistent Au(1); two of 22 other Au(1) hepatitis patients had persistent Au(1) Australia antigen was found in nine of 215 "normal" addicts (4.2%), significantly different from the general population (two of 2,412), but not different from 261 nonaddicts exposed to hepatitis by multiple blood transfusions (3.8%). Acute hepatitis in drug abusers is likely mainly viral, not toxic in origin.