Background: A large outbreak of hepatitis B virus (HBV) infection in the UK occurred between 2001 and 2005 in Bristol, UK. Objectives: To identify HBV strains circulating amongst risk groups in the HBV outbreak cohort. Study design: Cross-sectional study of acute HBV outbreak cases in Bristol. Results: HBV sequences from sera of 95 of the 237 cases (40%) were characterised. The majority of cases (77%) were found to carry an HBV variant belonging to genotype D, designated HBVBV. Eighty-eight percent (36/41) of sequences from injection drug users were HBVBV as were 70% (19/27) from those with heterosexual intercourse as the primary identified risk factor. Of 15 sequences characterised from cases of pre-outbreak acute or chronic hepatitis B residing in Bristol, 40% also carried HBVBV; the earliest was from a case identified in 1994. Conclusion: The findings from this study link the spread of HBVBV from injecting drug users to the general population through heterosexual intercourse during the outbreak. The molecular sequencing of specimens from this outbreak reports the emergence of HBVBV, a HBV strain circulating in Bristol and South West England, as the cause of one of the largest outbreaks of acute hepatitis B in the UK. © 2011 Elsevier B.V.