IPVS 2008: This paper describes an experimental design made to meet the challenges of getting sufficient and reliable data from a large amount of pigs. The design was used in a large clinical trial with vaccination against ileitis with Enterisol® Ileitis Vet. which was introduced in Denmark in August 2005. Comparison of efficacy reports collected from herds before and after vaccination showed an increased ADWG and a decrease in mortality and FCR after vaccination. However, such reports could leave doubts about whether the effect was due to the vaccine or originating from other changes made in the herds over time. A clinical trial was set up to confirm the results from the efficacy reports, and the experiences from this trial are described here.