Immunization against hepatitis B virus (HBV) is recommended for all sentenced prisoners and all new entrants to prison in the UK. In November 2008, acute hepatitis B was confirmed serologically in a 27-year-old man (Case 1) who had been incarcerated since February 2007. The cell mate of Case 1, a 26-year-old man was an established HBV carrier. A home-made tattoo gun was confiscated from their prison cell. In the absence of other clearly identifiable risk behaviours, tattooing was deemed to be a possible route of HBV transmission. Transmission of hepatitis B in a prison setting is a real concern and this report highlights the importance of immunizing prisoners against hepatitis B and should encourage health professionals to communicate the benefits of immunization to inmates to increase vaccine uptake.