Objective To explore the clinical application and safety of treatment of percutaneous radioactive 125I seed implantation treatment in lung metastases under CT guidance.Methods Twenty-seven lung metastatic malignancy cases (67 nodules) were studied.Eighteen cases (46 nodules) were hepatic cancer,4 cases (9 nodules) were prostate cancer,and 5 cases (12 nodules) were breast carcinoma.Diameters of lung nodules ranged from 0.5 cm to 4.0 cm with an average diameter of 2.1 cm.125I seeds were embedded under CT guidance,using 2 to 33 particles/nodule with an activity of 18.5 to 29.6 MBq/grain for each particle.Tumor matched peripheral dose was 90-120 Gy.Postoperative validation and quality evaluation followed.Results Four months later,24 nodules showed CR,30 showed PR,5 showed NC and 8 showed PD.The total effective rate was 80.6％ (54/67).In the course of treatment,11 patients had pneumothorax,3 had heavy lung compression and 4 had hemoptysis.All conditions were improved by pleural puncture or under close follow-up observation.Conclusion 125I seed implantation is an effective and safe technique in treatment of metastatic lung tumors under CT guidance.