The techniques of three-dimensional conformal radiotherapy (3 D-CRT) and patient immobilization have recently been developed, enabling us to focus high doses on the target with relatively less irradiation of normal tissues. In radiotherapy for solitary lung tumors, the local control may be safely improved by delivering a higher dose at only the target volume using these techniques. Recently, several clinical studies on stereotactic body radiotherapy (SRT) using the 3 D-CRT technique for solitary lung tumors have been reported. The single dose used is 10-15 Gy, and the total sessions are three to five. The local control rate is more than 90% and complication rates are very low. Therefore, this treatment is a promising new non-invasive treatment for early stage lung cancer. A multi-institutional clinical study, JCOG 0403, in now underway.