Levels of serum hepatitis B virus DNA-polymerase (HBV-DNAP) were studied longitudinally over variable periods of time in 16 HBV chronic carriers using a modified assay procedure developed to increase reproducibility. Ten patients were tested on a short-term basis at 3- to 6-hr intervals for 48 hr or at 24- to 48-hr intervals for 15 consecutive days, and all showed marked vacillations in enzyme levels although hospitalized and untreated. Patients with chronic active hepatitis on liver biopsy had larger fluctuations compared to cases of chronic persistent hepatitis. Six patients were longitudinally tested over a period of 12–32 months and those three receiving immunosuppressive drugs showed a progressive increase in DNAP levels during therapy, but two returned to pretreatment levels after therapy was withdrawn and one even cleared permanently the complete virus with seroconversion to anti-HBe. Such outcome was also observed in one patient with chronic active hepatitis who remained untreated.