Sera from 77 consecutive patients with acute type B hepatitis were examined for hepatitis B virus DNA (HBV DNA) by a spot hybridization method. The median follow-up time was 8 months (range, 1 week to 3 years). HBV DNA was detected in 26 (34%) patients on admission to the hospital. A significant positive correlation was found between short duration of symptoms and the presence of HBV DNA (p less than 0.025). Twenty-four (46%) of 52 HBeAg-positive patients were HBV DNA positive compared to 2 HBV DNA-positive patients of 25 HBeAg-negative patients (8%) (p less than 0.001). Four HBeAg-negative patients had serum HBV DNA initially or during follow-up; three had anti-HBe. Six of 77 patients with acute type B hepatitis (8%) became chronic HBsAg carriers, and HBV DNA was detectable from 5 months to more than 3 years after onset of symptoms. The presence of serum HBV DNA for more than 8 weeks after initial symptoms may predict development of a chronic HBsAg carrier state. In none of the chronic carriers was serum HBV DNA present after clearing of HBeAg.