Summary. Annual reporting of inhibitors to factors (FVIII) and IX (FIX) to the Canadian Haemophilia Registry has suggested a lower prevalence than that published in the literature. We performed a prospective study to determine the prevalence of patients with inhibitors directed against either FVIII or FIX. Patients with inhibitors were classified as: (i) inhibitor test positive; (ii) inhibitor test negative but on immune tolerance induction (ITI); (iii) inhibitor test negative but bypass treatment recommended; or (iv) inhibitor resolved. One year later, the cohort was re-classified. The prevalence of inhibitors on 1 May, 2007 was 3.3% for haemophilia A, 0.6% for haemophilia B and 8.9% and 2.1% for severe haemophilia A and B. One year later 17 individuals gained and 11 individuals lost inhibitor status (10 of these with ITI). This study suggests that the prevalence of inhibitors in our population is lower than that was previously published. We hypothesize that this is primarily due to the increased use of ITI, but other factors may be the unselected nature of the cohort and the restriction of the study to one date thereby conforming as close as practical to the definition of prevalence rather than incidence. The classification system used in this study was easy for clinics to apply and was important in defining the population with inhibitors.