IntroductionMethemoglobinemia is a condition in which methemoglobin is increased and the oxygen carrying capacity of tissues is decreased, causing a lack of oxygen to the whole body. RNA (ribonucleic acid) sequencing technologies have made it possible to systematically examine how the human transcriptome responds to invasive pathologies. To our knowledge, no previous studies have reported the results of RNA sequencing in a patient with methemoglobinemia. We describe the analysis of RNAs from the whole blood of a patient with methemoglobinemia.Case presentationA 31-year-old Japanese man was brought to our hospital with symptoms of dyspnea due to inhalation of gas from an acetic acid phosphonitrate storage tank at a factory. The nitrogen oxide concentration measured around the storage tank was over 2500 ppm, and he witnessed orange–brown smoke at that time. After entering the area and taking a few breaths, he suddenly became unwell, with dyspnea and numbness in his extremities. He was evacuated from the area within a few minutes, at which time he was suffering from whole-body cyanosis and was still aware of the above symptoms. On arrival at the hospital, his respiration rate was 18 breaths/minute, and his SpO2ranged from 80% to 85% on 15 L/minute of oxygen by mask (2.5 hours postexposure). Arterial blood gas testing revealed a methemoglobin level of 23.1%. After the administration of methylene blue, the patient’s methemoglobin level normalized and his symptoms improved. Chest X-ray and chest computed tomography showed no evidence of pulmonary edema or interstitial pneumonia, and no other abnormal findings were observed. RNA sequencing was performed on the blood samples obtained at the time of the visit, with the blood sample collected on day 5 used as a control. To our knowledge, the present study is the first to describe the analysis of RNAs from the whole blood of a patient with methemoglobinemia. The RNA sequencing analysis showed that an activated “hydrogen peroxide catabolic process” may be associated with the pathogenesis of methemoglobinemia.ConclusionThe results reported in the present study may explain the pathogenesis of methemoglobinemia.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13256-023-03976-0.