Hepatitis B virus (HBV) DNA sequences were assessed in 26 patients with acute type B hepatitis, using dot-blot hybridization technique from peripheral blood mononuclear cells (PBMC), during different phases of the illness. At clinical presentation, 15% of patients showed HBV-DNA sequences in PBMC, while serum HBV-DNA was detected in 58% of patients. During clinical improvement 50% of patients had HBV-DNA in PBMC but only 11.5% were positive for serum HBV-DNA. Twenty-three (88.5%) patients recovered and cleared HBV-DNA from serum and from PBMC; three (11.5%) patients with acute hepatitis progressing to chronicity showed persistently HBV-DNA sequences in serum and in PBMC. In conclusion, our study shows that HBV-DNA sequences may be found in PBMC, transiently in patients with acute hepatitis followed by recovery, persistently in patients with acute hepatitis progressing to chronicity.