An outbreak of hepatitis B originating in a family day nursery affected several children with Somali background. The transmission chain was confirmed by sequence analysis of the S gene. In Africa hepatitis B is often spread horizontally among children of pre-school age, a pattern of transmission that was retained in this outbreak. To limit the outbreak 126 children in the nursery and 50 members of staff had to be vaccinated. The total cost for this intervention was estimated to about 300,000 SEK. Considering the great number of immigrants in Sweden from areas highly endemic for hepatitis B the inclusion of vaccination in the general child immunisation program seems to be the most cost effective measure for long term prevention not only of hepatitis B transmission among children but also of venereal spread in early adulthood.