58 patients with acute hepatitis type B, including 13 with fulminant hepatitis, were tested on presentation for HBeAg. Positive results were obtained in 24%. The frequency was highest in those with fulminant hepatitis (46% positive), and this was probably related to the earlier presentation of patients with this condition. Patients with acute hepatitis who were HBeAg-positive had had a significantly shorter duration of symptoms than those who were HBeAg-negative at presentation. HBeAg was still detectable more than 3 weeks after the onset of symptoms in only 2 patients, both of whom progressed to chronic active hepatitis. The early detection of HBeAg in patients with acute hepatitis is of no prognostic significance, but its persistence may provide the earliest evidence of potential chronicity.