Hepatitis D virus is a defective human pathogen that requires hepatitis B virus for its replication. A hybridization-based assay for the 1.75 kb RNA genome of hepatitis D virus was developed using as probe a radiolabeled transcript of a cloned cDNA fragment (pKD3 hepatitis D virus DNA). Sera from 120 chronic carriers of HBsAg with confirmed hepatitis D virus infection were analyzed for the presence of hepatitis D virus RNA. Serum hepatitis D virus RNA was detected in 43 of 74 (58%) patients with chronic liver disease; some patients were positive for hepatitis D virus RNA in multiple samples over a period of several years. Serum hepatitis D virus RNA was present in 17 of 28 (61%) patients during the acute phase of clinical hepatitis and was not detected after recovery from acute disease or in 18 asymptomatic chronic HBsAg carriers with antibody to hepatitis D virus. The presence of hepatitis D virus RNA correlated with other known markers of active hepatitis D virus replication; all chronic active liver disease patients with serum hepatitis D virus RNA were positive for antihepatitis D antigen IgM, and 34 of 37 (92%) had hepatitis D antigen in their liver biopsy specimens. The assay for hepatitis D virus RNA provides a direct and noninvasive method for the detection of hepatitis D virus in serum and will be useful in the study of the natural history of type D hepatitis, the identification of chronic hepatitis D virus carriers likely to transmit hepatitis D virus and the selection and monitoring of patients for potential antiviral therapy.