We report a case of unexpected severe skin necrosis after autologous transverse rectus abdominis musculocutaneous flap breast reconstruction in a patient with homozygosis for the 5,10- methylenetetrahydrofolate reductase (MTHFR) gene. This genetic deficiency is hypothesised as the cause of this exceptional skin necrosis. A 46-year-old woman underwent a radical mastectomy and immediate rectus abdominis musculocutaneous flap breast reconstruction. At the end of surgery, the blood supply to the flap and the abdominal wall was excellent. On the 5th postoperative day, the patient developed an extensive abdominal skin necrosis and a partial flap necrosis on the reconstructed breast. The rectus abdominis musculocutaneous flap breast reconstruction can be proposed to patients without any haemostatic defect in order to avoid life-threatening complications and unaesthetic results. This procedure requires careful patient selection, detailed preoperative planning, and complete laboratory investigations: However, mutation of the 5,10- methylenetetrahydrofolate reductase gene is too exceptional to form part of the routine preoperative investigation and can be looked for in these cases of extensive necrosis.