Factors VIII and IX administration exposes haemophiliac patients to the risk of inhibitor development which reduces treatment efficacy. Methods and results of 17 studies (transversal and longitudinal) and of one clinical trial dealing with haemophilia A and of 3 studies dealing with haemophilia B were reviewed. Besides differences in study design, lack of method standardisation complicates comparison between studies. In patients with haemophilia A, prevalence of inhibitors ranges from 4 to 18% (5 studies), and incidence between 3 and 39/1,000 person-years (7 studies). Cumulative age incidence is above or equal to 20% after 6 years of age (4 studies). Cumulative incidence by exposure days is estimated at 22% at 100 exposure days in a retrospective cohort observed between 1975 and 1992, and above or equal to 36% at 25 exposure days in two prospective cohorts of patients receiving recombinant factor VIII products. The new European regulation dealing with stable blood products will make the evaluation of new products by clinical trials mandatory. With The National therapeutic Follow up of Haemophiliacs which begun in October 1994, following a decision by the French Minister of Health, the adverse effects in patients receiving factor VIII and IX which have been approved for sales will be evaluated.