The objective of the present study was to investigate risk factors associated with the introduction of acute clinical infectious bursal disease (IBD) among Danish broiler chickens in 1998. Data on 218 flocks were collected from hatcheries, abattoirs, farmers and veterinarians; 49 of the flocks had experienced acute clinical IBD (cases), 169 were unexposed (controls). The study was carried out using a case-control design. Cases were defined as the first flock on each premises to experience acute clinical IBD, and these were compared with non-diseased, non-IBD-vaccinated control flocks chosen randomly from each unaffected farm. The resulting numbers of cases and controls used for statistical analyses were 16 and 61, respectively. Statistically significant associations were seen between the initial 16 Danish cases of acute clinical IBD in 1998 and certain hatcheries, age of parent birds and a certain feed mill.