Serum levels of hepatitis B virus specific DNA polymerase and hepatitis B e antigen were studied serially in 34 patients with hepatitis B virus infection--20 who had the acute illness and recovered, seven who died with fulminant disease, three who died as a result of subacute hepatic necrosis, and four who went on to develop chronic active hepatitis. DNA polymerase activity was present in 16 (80%) and HBeAg in 13 (65%) of the uncomplicated cases at presentation and in all of those patients from whom the initial sample was obtained before the peak in aminotransferase. Both markers disappeared after 30 days from the onset but DNAP remained persistently positive during a follow-up period of four to 10 months in the four patients who progressed to chronic hepatitis. These results indicate that DNAP and HBeAg are transiently present in all cases of acute hepatitis B. Only their persistence after the acute episode could represent a useful prognostic marker of chronically. In this respect, DNAP was more reliable in our patients than HBeAg. In uncomplicated acute hepatitis, the peak in DNAP levels, which defines the time of maximum virus replication in the liver, preceded the peak in aminotransferase levels. Among the 10 patients who developed massive liver damage after hepatitis B infection, DNAP was detected in five of the seven with fluminant hepatitis, with enzyme levels that were comparable with those observed in uncomplicated acute hepatitis and presentation, but not in the cases of subacute hepatic necrosis. These findings are consistent with the hypothesis that in hepatitis B infection, liver damage, whatever the severity, is not directly related to the degree of virus replication.