Methemoglobinaemia is a rare blood disorder that affects the ability of haemoglobin to transport oxygen around the body due to the presence of Methemoglobin. Patients may be asymptomatic, but in some cases, it may be life threatening [1]. Methemoglobinaemia can be congenital or acquired [1]. This is a case report of a 39-year-old man who developed acute methemoglobinemia postoperative day 1 for a tibial fracture. The patient presented with an acute severe desaturation to 74% on room air. He remained markedly cyanosed and tachycardic despite receiving 15L high-flow oxygen therapy. Arterial blood gas analysis revealed elevated methaemoglobin levels and a characteristic dark brown coloration of the blood (Figure 1). Prompt recognition of methemoglobinaemia led to rapid improvement in the patient's condition. This case highlights the importance of considering methemoglobinaemia in the differential diagnosis of hypoxemia and emphasises the need for timely intervention to prevent adverse outcomes.