One hundred forty-four serum samples from 52 patients with chronic hepatitis D virus infection were analyzed for hepatitis D virus RNA by dot-blot hybridization using hepatitis D virus cDNA probe labeled with 32P. The results were correlated with the presence of serum IgM anti-hepatitis D virus and hepatitis D antigen in liver biopsy specimens when available. Although there was a trend of positive correlation between serum hepatitis D virus RNA and IgM anti-hepatitis D virus, no statistical significance could be found. In the serum samples with hepatitis D virus RNA, 32% were found to be negative for IgM anti-hepatitis D virus. Therefore, in chronic hepatitis D virus, absence of IgM anti-hepatitis D virus does not rule out active viral infection, as suggested by previous studies. There was a strong correlation between serum hepatitis D virus RNA and hepatic hepatitis D virus antigen. These data indicate that detection of hepatitis D virus RNA in serum samples is a reliable noninvasive marker of active viral infection.