We have investigated the usefulness of serum hepatitis delta virus (HDV) RNA detection using a slot hybridization analysis of serum samples from ten patients with acute hepatitis and delta markers (group I), from 28 patients with chronic delta hepatitis (group II) and from seven liver graft recipients with hepatitis B virus (HBV) and HDV related cirrhosis or fulminant hepatitis (group III). The slot-blots were hybridized with both HDV-complementary DNA and single-stranded RNA probes. With the single-stranded RNA probe, HDV RNA was detected in the first serum sample available in 9/10 of the patients with acute hepatitis (group I). In addition, HDV RNA was detected in 8/9 and 7/8 of the samples obtained within and after 1 month of the onset of hepatitis. Five of the ten patients scored positive for HDV RNA and negative for hepatitis delta antigen (HDAg) while one was negative for HDV RNA and positive for HDAg. The same RNA probe enabled the detection of serum HDV RNA in 21/28 chronic hepatitis patients (liver HDAg and/or IgM anti-HD positive) (group II). Among the liver graft recipients (group III), 7/7 had a recurrent delta infection. Serum HDAg, liver HDAg and anti-HD IgM were identified in 3/7, 6/7 and 5/7 of the patients, respectively. HDV RNA was detected in the seven patients with either persistent (4/7) or transient (3/7) positivity. In addition, HBsAg and HBV RNA were persistently shown in 4/7 patients with continuous HDV replication. In the remaining three patients, HDV RNA was detectable despite the absence of HBsAg.(ABSTRACT TRUNCATED AT 250 WORDS)