Summary. Inhibitor development, because of its impact on patients’ morbidity and quality of life, is presently the most serious complication of haemophilia A treatment. The identification of several genetic and non-genetic risk factors may be used for the stratification of inhibitor risk and the definition of prevention strategies, particularly for patients with a high-risk genetic profile. The most extensively studied genetic factor is the type of F8 mutation, i.e. large deletions, nonsense mutations and inversions, which are associated with a higher risk of inhibitor development. This is the basis for the increased risk in patients with inhibitor family history; however, concordance family studies showed that factors other than F8 mutations are involved. An emerging role is investigated for polymorphisms of immune-regulatory genes that may increase (IL-10 and TNF-α) or reduce (CTLA-4) inhibitor risk and whose heterogeneous ethnic distribution may correlate to the higher inhibitor risk in non-caucasian patients. A role for FVIII haplotypes, particularly in black haemophiliacs, has been recently proposed. Recent studies report an increased inhibitor risk for initial intensive treatments (surgery or severe bleeds requiring high-dose and/or prolonged treatment, presence of danger signals), whereas regular prophylaxis (absence of danger signals) exerts a protective effect. A clinical score including the type of F8 mutation, family history of inhibitors and intensive treatment has been recently validated for predicting inhibitor risk. Because of the lack of useful data regarding the role of different types of FVIII concentrates, the stratification of risk in patients starting replacement treatment together with the careful evaluation of indications, doses and duration of treatment at first exposures and further efforts for overcoming barriers to early implementation of prophylaxis are encouraged, particularly for patients with a predictable high inhibitor risk.