We report a rare case of progressive severe pulmonary failure due to exposure to dimethyl sulfate vapor. The patient is a 48-year-old man who carried four bottles of dimethyl sulfate on the carrier of a truck and one of them was broken. After he had been cleaning up the carrier for an hour while covering his nose and mouth with a dry towel, he had felt pain of his eyes, pharyngolarynx and nose. He went to an emergency hospital on foot 3 hours after exposure, because he felt obstruction of the pharyngolarynx, hoarseness and dyspnea. He was immediately intubated due to severe hypoxia and was immediately administered hydrocortisone intravenously, and methylprednisolone inhalation therapy was prescribed for ten days. Tracheostomy was performed the 5th day following the onset of symptoms because of severe laryngeal edema. Sputum and strider continued after steroid therapy was suspended because of Pseudomonas aeruginosa infection. The steroid therapy was resumed as transbronchial lung biopsy suggested peribronchiolar inflammation with granulation change, and he improved promptly. However, he had another bout of pneumonitis thereafter, and dyspnea and hypoxia gradually developed. Home oxygen treatment was introduced the 5th year after onset, and then roentgenogram showed severe emphysematous change, and now he is listed for lung transplantation. We ewcommend a high dose of steroid in the acute phase and a low dose of erythromycin for a long term in case of lung injury due to exposure to dimethyl sulfate.