Extramedullary hematopoiesis (EMH) is a rare finding in hematology. A 73-year-old female patient with a 1-week history of severe progressive dyspnea was examined, and computed tomography (CT) showed a paratracheal mass 3 cm in size located 1 cm below the vocals cords and causing obliteration of the tracheal airway. Cytology of a needle biopsy revealed EMH. External radiotherapy of 200-cGy fractions to a total dose of 2000 cGy was administered with 3-dimensional conformal planning to treat the progressive symptoms. The patient's clinical symptoms started to improve 2 days after radiotherapy and had completely disappeared after 7 days. CT scans showed complete response on follow-up at 1 week to 5 months after radiotherapy. Mature and immature hematopoietic cells and many adipose cells were seen in the pretreatment samples. Histologic findings in the posttreatment samples showed that these cells had completely disappeared due to the conformal radiotherapy. On the basis of clinical, radiologic, and histologic results, we suggest that conformal radiotherapy may be useful for the treatment of paratracheal localization of EMH because good tumoral irradiation was obtained in this case, with the protection of normal tissues. Int J Hematol. 2001:73:492-495. (C) 2001 The Japanese Society of Hematology.