We present the case of an adult patient presenting with clinical features suggestive of asthmatic bronchitis due to respiratory tract infection and a low pulse oximetry reading, who was finally diagnosed suffering from hereditary methemoglobinemia. Presence of a wide discrepancy in haemoglobin oxygen saturation, as assessed by pulse oximetry (SpO₂) and arterial blood gas analysis (SaO₂), along with normal arterial blood partial pressure of oxygen (PaO₂) are findings highly suggestive of elevated methemoglobin concentration. Measurements of SpO₂ and SaO₂ are, due to technical reasons, unreliable in cases of, either hereditary or due to exposure to an oxidizing agent, Methemoglobinemia and alternative methods estimating tissue oxygenation have to be used.