Inhibitors are more common than previously thought in any population of patients with haemophilia, particularly those with severe haemophilia. Comparison of a recent analysis of retrospective data on inhibitor incidence with data derived from recombinant factor VIII studies suggests that the incidence of inhibitors is not substantially different between the two groups. However, a distinction must be made between high- and low-responding inhibitors and it is not clear whether low-level inhibitors in some patients are part of the natural history of haemophilia. Questions remain on why some groups might differ in inhibitor levels. Further work is required to ensure standardization of future protocols and continued observation of present cohorts.