Sera from 79 patients with acute self-limiting hepatitis, 17 patients with acute hepatitis B evolving into chronic HBsAg carriership, and 43 chronic HBsAg carriers without a history of acute hepatitis were analyzed for presence of hepatitis B virus (HBV)-DNA by a molecular hybridization technique. In acute self-limiting hepatitis, HBV-DNA was cleared within a few weeks after the onset of clinical symptoms. The longest period of DNA positivity observed in this group was 42 days. In 29 of 52 patients HBV-DNA was cleared before HBeAg disappeared. Among 17 patients who became chronic HBsAg carriers, HBV-DNA was present for more than 6 months in all but one. Most of the HBsAg carriers eventually cleared HBV-DNA. The DNA clearance frequently preceeded the conversion of HBeAg to anti-HBe. Thus, in many patients there was a transitional period with HBeAg but without HBV-DNA. HBV-DNA was found to be a better index of impending chronicity than HBeAg since persistence of HBeAg for more than 42 days was noted in 10% of the patients who nevertheless cleared HBsAg within 6 months. By that time all those patients had turned negative for HBV-DNA. On the other hand, in 16 of the 17 patients who became chronic carriers of HBsAg, HBV-DNA as well as HBeAg persisted for more than 6 months. The present results also suggest that infectivity in acute hepatitis B is a feature mainly of the presymptomatic and early symptomatic period.