Emicizumab (MC-Ab) is a humanized bispecific antibody which binds to factors IX-activated and X, speeding up the activation of factor X. It solved some unmet needs in hemophilia A (HA) treatment, such as regimen (once weekly up to once monthly infusion) and route of administration (subcutaneous). Although it is an effective non-replacement alternative in the prophylaxis of people with HA and inhibitor (PwHAi), its safety has not been clarified yet. It was approved in 2018 for prophylaxis of children and adults with HA in Brazil, despite inhibitor (anti-factor VIII [FVIII] antibodies) status. The development of inhibitor in non-severe PwHA can be as dangerous as in severe PwHA because the bleeding rate increases considerably, and hemostasis with bypassing agents is not as predictable as with FVIII. Herein we report the first woman with PwHAi included in the “Brazilian registry of persons with hemophilia A receiving emicizumab”(Emicizumab Cases, EMCase Project). The patient is a 28-year-old brown woman who was diagnosed as mild HA (FVIII activity 10.0%) when she was 9 years. She had been treated in another service, due to Melnick-Needles syndrome (a genetic disorder of bone characterized by skeletal and cranio-facial abnormalities). She had frequent purpura and arthralgia/arthritis treated with physical therapy, analgesics and, sometimes, corticosteroids. In 2010, due to a slight increase in the partial thromboplastin time, mild HA was diagnosed. She was transferred to the Hemophilia Treatment Center of Paraiba (HEMOIBA), when on-demand FVIII was started. Peripheral venous access was considered extremely difficult to provide due to body structure. She developed a high-response inhibitor two years later. Immune tolerance induction was not tried because a difficult peripheral venous access. Central venous access was postponed due to lack of a Vascular Surgeon. She was kept on activated recombinant factor VII (rFVIIa) episodic treatment, with an annualized bleeding rate (AzBR) of 9.9 episodes/y. Between Jan/28/2019 and Jan/28/2020 (pre-MC-Ab period), the AzBR was 11.0 episodes/y. rFVIIa consumption was 18,750 kIU, both for bleeding episodes and intermittent prophylaxis. On Jan/28/2020, after rFVIIa was withheld, she received MC-Ab attack of 3.0mg/kg once weekly for 4 weeks. MC-Ab was maintained as 1.5mg/kg weekly. She self-infused MC-Ab at home. From Jan/28/2020 to Jul/28/2020 (MC-Ab period), AzBR was 0.0 episode/y. She did not receive MC-Ab during 5 consecutive weeks on May-Jun/2020 and 2 consecutive weeks on Jul/2020. The estimated annual consumption of MC-Ab was 2,340mg, including attack+maintenance doses, or 2,160mg, excluding attack doses, both considering AzBR of 0.0 episode/y. The estimated annual cost of treatment increased from US$ 216,187.00 (US$ 11.53/kIU rFVIIa, in 2018) to US$ 187,342.20-202,954.05 (US$ 86.73/mg MC-Ab, mean of the 4 purchases). Puncture site adverse events and thrombosis event were not reported. Prophylaxis with MC-Ab was effective and safe. Based on our estimated annual direct costs, it can save up to 15% compared to rFVIIa on-demand treatment.