On the basis of published sequence data the preS1 attachment region of hepatitis B virus (HBV) appears to be highly variable. Using a novel method for rapid DNA sequencing by the polymerase chain reaction we screened 34 HBV DNA-positive sera for mutations in a variable part of the preS1 region of the HBV genome. The sequence data were used to analyse potential chains of infection, and strongly supported the expected routes of HBV transmission among patient groups. Furthermore, sequence comparisons permitted sub-genotyping of the viruses. In the 22 cases of subtype adw, we found a very low number of point mutations. This shows that the attachment site of HBV is more highly conserved than that of other blood-transmissible viruses such as human immunodeficiency virus or hepatitis C virus.