In 2001, two residents of a nursing home in Lower Saxony, Germany, were diagnosed with acute hepatitis B virus (HBV) infection. A systematic contact investigation of 188 residents yielded 19 confirmed or probable cases of acute or recent HBV infection and three persistent asymptomatic HBsAg carriers. Sequence analysis revealed that one carrier had high viraemia (109 genomes/ml), HBV genotype A2, and the same S gene and/or X gene sequence as 16 acutely infected persons. An unmatched case-control study was conducted with the 17 cases that had sequence identity together with 26 controls. The strongest association was found for treatment by a particular general practitioner (GP) (OR>11, P<0·001) and blood sampling for glucose monitoring on a particular day by the GP's staff (OR 13·6, P<0·001, adjusted OR 8·5, P=0·017). Control measures were implemented. Serological controls after 6 and 18 months revealed that the outbreak was brought under control.