Following an outbreak of hepatitis B virus (HBV) infection amongst immunosuppressed transplant recipients, the complete sequences of the HBV-DNA isolated from nine of the affected patients were determined. The DNA sequences were found to differ from each other by a maximum of three nucleotides and belonged to the same serotype (ayw3). By contrast, the sequences differed by 18 nucleotides from the most similar HBV-DNA sequence published, indicating a common source of infection. The infection chains that have been constructed according to the base differences between the DNAs agreed well with those previously established on the basis of epidemiological data Journal of Hospital Infection 26:81-92]. At least two HBV populations, differing by one or two nucleotides, were detected in four patients, and coexisted for differing periods of time. Mutations of the core and X-peptide were not found. The data were used to calculate evolution rates of HBV DNA, both for HBV persisting within a patient and for infection chains. The rates obtained were of the same order as described previously for immunocompetent patients, indicating that the immunosuppressive medication did not influence the evolution rate. However, the evolution rate was found to decrease with increasing evolution time.