Particular epidemiological features of Mycobacterium bovis infection in cattle herds in Great Britain during the period 1972-8 were examined. During these seven years 1099 herds became infected, the mean annual incidence of herd infection being of the order of one infected herd per 1000 cattle herds. Infection in herds was predominantly a sporadic occurrence; 938 (85.4%) herds experienced only one incidence of infection which persisted for less than 12 months. The concentration of infected herds in localized areas of the south-west region of England, where infected badgers were the most significant attributed source of infection, is demonstrated. The risk of herd infection in relation to badger sett density was also examined in Cornwall, Gloucestershire/Avon and counties in England and Wales outside the south-west region of England. The numbers of herds at risk in six categories of badger sett density in these three areas were estimated from three random samples of herds drawn from the annual agricultural census. In Cornwall and Gloucestershire/Avon herd infection, associated with infected badgers or for which no source of infection could be found, was positively associated with badger sett density. A similar association between herd infection, not attributable to a source of infection, and badger sett density was found in counties in England and Wales outside the south-west region of England.