In 3 clusters of postsurgical hepatitis B virus (HBV) infection, HBV DNA sequence mismatches were observed between the transmitting surgeons and the patients whom they infected. Sequence analysis of clones amplified from the C gene of HBV suggested that the mismatches were due to transmission of a minority variant in the circulation of each surgeon. Compared with 5 other transmitters from whom transmission of the dominant variant was demonstrated, the 3 surgeons who transmitted minority variants carried significantly more heterogeneous HBV populations. Transmission of minority variants was not correlated with the transmitters' hepatitis B antigen status, the presence of the position 1896 precore mutant, or the level of HBV viremia. In 1 cluster, a variant comprising <10% of the HBV population circulating in the transmitting surgeon established infection in all 3 patients who acquired HBV through him, which substantiates the phenomenon of true selection.