Investigation by polymerase chain reaction of HBV-DNA in serum from chronic hepatitis B virus carriers is not widely used for routine diagnosis because polymerase chain reaction assays are complex and may be too sensitive. We investigated the sensitivity, the specificity and the possible value for clinical use of a simplified polymerase chain reaction method in which a single, 30 cycles round of polymerase chain reaction is performed using only 10 microliters of serum treated with microwaves. The efficiency of the polymerase chain reaction in amplifying HBV-DNA was greater after microwave irradiation of serum than after alkaline extraction, but lower than after protein digestion and phenol chloroform precipitation. Despite its simplicity and high sensitivity, the assay was very specific. Studies in anti-HBe positive chronic hepatitis B virus carriers demonstrated HBV-DNA sequences in 1/15 (7%) healthy carriers, in 4/20 (20%) patients with slight alanine aminotransferase elevation, in 16/18 (89%) with marked alanine aminotransferase elevation and in all 20 with fluctuating alanine aminotransferase levels. In the latter, HBV-DNA was detected either at exacerbation (two cases), during remission (one case) or both (17 cases). HBV-DNA was detected by classical dot-blot hybridization in only 24/58 (41%) samples that were positive by the simplified polymerase chain reaction method. Although extremely high sensitivity is not achieved, microwave irradiation of serum simplifies considerably the detection of small amounts of HBV-DNA and makes polymerase chain reaction suitable for monitoring patients in whom weak hepatitis B virus replication is associated with ongoing liver disease.