Antibodies that block factor VIII (FVIII) activity appear in some haemophilia A patients treated with FVIII replacement therapy and severely impaired treatment. To date, the mechanisms that lead to this immune response are unknown. In this work, haplotypes of cytokine interleukin 10 (IL-10) gene have been associated with the presence of FVIII inhibitors in a group of Brazilian haemophilia A patients. The coexistence of a haplotype defining high IL-10 synthesis and one defining an intermediate production of cytokines is found to be associated with the group of patients who have a history of inhibitor development. Additionally, the coexistence of haplotypes defining high and low IL-10 syntheses is strongly associated with the group of negative inhibitors. These results have shown that the simple association considering only the presence or the absence of a haplotype and the development of inhibitors in haemophilia A is not sufficient. Data obtained in this work sustain the idea that the genetic studies may partly explain why only approximately 25% of haemophilia A patients develop FVIII inhibitors. Other genetic issues and details of the protein replacement therapy should be considered to measure the chances of a patient to develop anti-FVIII antibodies.