The hepatitis D virus (HDV; previously called the "delta agent") is a defective organism which can replicate only in the presence of the hepatitis B virus (HBV). We tested the serum of 95 substance abusers, all of whom had sufficient evidence of chronic liver disease to warrant a liver biopsy, for hepatitis D virus antibody (anti-HDV). Anti-HDV was detected in five of eight hepatitis B surface antigen (HBsAg)-positive patients and 12 of 87 (14%) HBsAg-negative patients. Antibody to the hepatitis B core antigen (anti-HBc) was the sole hepatitis B marker in eight of the 12 (67%) anti-HDV-positive, HBsAg-negative patients but in only 14 of 75 (19%) anti-HDV-negative, HBsAg-negative patients (P less than 0.005). None of the anti-HDV-positive, HBsAg-negative patients had detectable IgM anti-HBc in the serum or hepatitis D antigen in liver tissue, and they had similar clinical features and liver biopsy diagnoses to HBsAg-negative patients without anti-HDV. We conclude that anti-HDV in HBsAg-negative substance abusers reflects infection with HDV and HBV in the distant past and does not indicate more severe liver disease than that seen in HBsAg-negative patients without anti-HDV.