An 83-year-old woman was admitted to our hospital because of fever and difficulty in walking with marked hypoxemia and diffuse ground glass opacities in bilateral lung fields by chest radiography and CT scanning. Treatment with antibiotics and corticosteroids resulted in improvement of the clinical findings and successful weaning from mechanical ventilation. At first, we diagnosed severe mycoplasma pneumonia because of a 2560 titer in a particle agglutiation (PA) test. Five days after discharge, she was given a second emergency admission, because of fever and difficulty in walking. Chest X-ray film revealed improvement after administration of methylprednisolone. Her detailed medical history proved that she had been treated with a Chinese herbal drug, Sai-rei-to, for several weeks before first admission. We finally diagnosed her disease as Sai-rei-to-induced pneumonitis. Despite intensive treatment, she finally died. The histopathological findings (H-E stain) of the autopsied lungs showed hyaline membrane formation and hyperplasia of type II alveolar epithelium cells, so-called, diffuse alveolar damage. This case and other referred in the literature suggest that Sai-rei-to-induced pneumonitis can become severe.