To evaluate the profile of hepatitis D virus replication and the corresponding immunoresponse after acute hepatitis D virus infection, sera from 50 patients with acute hepatitis D (36 with acute hepatitis B virus-hepatitis D virus coinfection and 14 HBsAg carriers with hepatitis D virus superinfection) were investigated for the presence of hepatitis D virus RNA and other serological hepatitis D virus markers. During the first week after onset of symptoms, hepatitis D virus RNA was detected by spot hybridization with a similar frequency among patients with coinfection (64%) and those with superinfection (71%). The presence of hepatitis D virus RNA in the first serum sample correlated with that of circulating hepatitis D antigen in both groups of patients. The presence of hepatitis D virus RNA was transient and its clearance paralleled that of serum hepatitis D antigen among patients with coinfection, so that 1 month after the onset of symptoms serum hepatitis D virus RNA was no longer detectable in any of these patients. Conversely, serum hepatitis D virus RNA was still present in 78% of those with superinfection, all of whom developed chronic liver disease, thus suggesting that the persistence of hepatitis D virus RNA in the serum for more than 4 weeks might indicate progression to chronicity. In nine of the 14 patients (64%) with hepatitis D virus superinfection progressing to chronicity, hepatitis D virus RNA was persistently detected throughout the follow-up, whereas in five patients it was detected occasionally. In four superinfected patients hepatitis D virus RNA and hepatitis B virus DNA were detected simultaneously in serial samples, thus suggesting that, at least during early stages of chronic hepatitis D virus infection, both viruses may replicate at the same time.