Tc-99m MIBI, a lipophilic cation, was reported as a useful agent for localization of lung cancer. The effect of radiation therapy on the uptake of Tc-99m MIBI in lung cancer, however, was not well evaluated. The aim of the present study was to elucidate the usefulness of Tc-99m MIBI SPECT in the localization and the assessment. of radiotherapy in non-small cell lung cancer, Tvrenty patients(19 males and 1 female, mean age 59, 16 squamous cell ca and 4 adenocal were studied with Tc-99m MIRI SPECT before radiation therapy. Eleven patients(10 males and 1 female, mean age 59, 8 squamous cell ca and 3 adenoca) were repeated the study 1 month after the completion of radiation therapy(mean dose 6453cGy). All patients showed positive uptakes of Tc- 99m MIBI in their tumors. One patient showed a hot uptake in atelectatic area. There was no difference of Tc-99m MIBI uptakes between squamous cell ca and adenoca either on planar or tomographic images. Tc-99m MIBI uptake ratios of squarnous cell ca and adenoca were 1.50 +- 0.16 and 1.45 +- 0.15 on planar images, and 2.73 +- 0.46 and 2.54 +-0.37 on tomographic irnages, respectively. The concordance between radivlogical change(chest x-ray and CT) and change of Tc-99m MIBI uptakes was 9/11 (81.8%). In conclusion, Tc-99m MIRI SPECT was useful in the localization of tumor and the assessment of radiation therapy in non- small cell lung cancer.