Both genetic factors and environmental factors are suggested to play a role in the aetiology of inhibitor development in patients with severe haemophilia A. Monozygotic twins are ideal candidates to study the influence of environmental factors. We describe a pair of 3-year-old monozygotic twin brothers with severe haemophilia A. Prenatal diagnosis confirmed the presence of an intron 22 inversion. Other patient-related factors such as birth weight, vaccinations and the duration of breastfeeding were similar. At the age of 7 months, one boy suffered from a spontaneous subdural haematoma, which needed complete correction of haemostasis with continuous infusion of a third-generation recombinant factor VIII. A persistent high-titre inhibitor with severe clinical symptoms developed, that could only be eradicated with high-dose immune tolerance induction (ITI) for 36 months in combination with rituximab therapy. His twin brother first received treatment at 9 months of age with the same FVIII product. After treatment on nine exposure days, he developed a low-titre inhibitor at the age of 14.5 months. Unlike his brother, he was tolerized without difficulties with low-dose ITI within 2 months. The discordant antibody responses were underlined by dissimilar IgG1 and IgG4 levels in their plasma. The discordant immune response to FVIII in this pair of monozygotic twin brothers seemed to be related to intensity of treatment and severity of bleeds. This confirms that these environmental factors play an additional role in the development of inhibitors.