The situation of radiotherapy in Thailand from its beginning to the present is reported. The situation of radiotherapy in other countries of South East Asia is also analyzed from the data or information requested through the Radiological Society of each country. The situation of radiotherapy in each country will be discussed in terms of equipment, facilities, personnel, teaching and training, research, and clinical practices. In each country, efforts have been made to develop radiotherapy facilities so that services can be given at a modern standard. Some countries has attained that standard and full range of radiotherapy has been available. An effort has been made to help each other and exchange knowledge by organizing the Asian and Oceanian Society of Radiology (AOSR) and the Asian Association of Radiology (AAR) through which Congress of Radiology has been organized every 2 alternate years. © 1990.