Results with the polymerase chain reaction and conventional DNA hybridizing technique (dot-blot) for the detection of hepatitis B virus (HBV) DNA were compared for 439 patients. In 261 patients who were positive for HBs antigen (Ag), HBV DNA was demonstrated with the polymerase reaction in all of the 69 HBe-Ag positive sera (dot-blot 56%), as well as in 81 (62%) of 131 anti-HBe positive sera (dot-blot 5%), in 29 (48%) of 61 HBe marker negatives (dot-blot 3%) and in six (29%) of 21 delta positive sera. Among HBs-Ag negative patients, HBV DNA was detected in six (22%) of 27 anti-HBc positive sera, but not in sera (n = 50) which were both anti-HBc and anti-HBs positive, as well as in HBV marker negative patients with liver disease (n = 30) and healthy controls (n = 50). If infectiousness is to be checked or in case of double infection, the more sensitive polymerase chain reaction is to be recommended for the detection of HBV DNA. HBe-Ag positive patients must be considered as infectious: tests for HBV DNA can in general be omitted.