An outbreak of hepatitis B virus (HBV) infection in a haemodialysis unit is described. Four patients in the unit contracted subclinical HBV infection within three months. DNA sequence analysis of the S gene of HBV isolates from chronic carriers and newly infected patients in the unit aided in tracing possible transmission pathways. Three newly infected patients had received partial or complete HBV vaccination previously. HBV was rapidly cleared from all three although the anti-HBs titre had not reached 10 IU L-1 in any of them at the time of infection.