A survey of the infectious bursal disease (IBD) status of 85 clinically normal broiler flocks was carried out. Flocks were grouped into three categories: flocks without IBD lesions (category A), flocks with typical acute IBD lesions (B) and flocks with typical chronic IBD lesions (C). Category A flocks achieved a net income per 1,000 birds which was 11% better (P<0.05) than category B flocks and 14% better (P<0.01) than category C flocks. The 85 broiler houses used by the surveyed flocks during production were similarly grouped into the three categories. Analysis of production data from 991 flocks comprising 14 million birds demonstrated that flocks reared in category A houses achieved a similar superiority in net income per 1,000 birds over those flocks reared in both category B and category C houses. Food conversion ratio and average weight per bird were also superior in flocks reared in category A houses. These findings suggest that IBD virus persisted from flock to flock in category B and category C houses. The differences in performance were greatly accentuated during the winter months. The results indicate that the presence of IBD virus infection accounts for about two-thirds of the reduced profitability achieved by broiler flocks in Northern Ireland during the winter.