Reactivation of chronic hepatitis B is characterized by the reappearance of HBV-DNA in serum. The purpose of the study was to determine whether, before reactivation, HBV-DNA would be detectable in serum, using a sensitive procedure of detection, namely polymerase chain reaction (PCR). We studied 17 patients with chronic hepatitis B who experienced an episode of reactivation, defined by the reappearance of HBV-DNA in serum. None of these 17 sera had HBV-DNA demonstrable by dot-blot hybridization nor liquid hybridization in sera collected before reactivation. Using PCR, HBV-DNA was detected, before reactivation, in 13 of the 17 episodes of reactivation tested with Southern-blot and hybridization. HBV-DNA was not detectable with PCR in the serum of four patients who subsequently experienced an episode of reactivation. In conclusion, our results show low level HBV replication before reactivation in most, but not all, HBs-positive, HBV-DNA-negative patients. This suggests that reactivation may occur even in patients with no HBV-DNA demonstrable in serum with PCR prior to reactivation.