Seven cases of lung cancer, five of them being so-called small sized, were treated with Bleomycin preoperatively and following results were obtained. 1) In X-ray studies the reduction of tumor size was obtained in three cases, the thinning of tumor shadow in two cases, and the change of shadow configuration in two cases. 2) In histological sections, the degeneration of cancer cells and bizarre cells were seen in all cases. Moreover coagulation necrosis and fibrosis of cancer tissue were also seen in all cases. 3) As to the influence of Bleomycin to the normal lung tissue, the swelling of alveolar cells, the widening of alveolar septa and interstitial edema of alveoli as well as the dilation and edema of the alveolar capillaries and sparseness, swelling and thickening of vascular wall were seen. These capillary and vascular changes showed a parallel relation to the total dose of Bleomycin. 4) Six patients, who were operated, have survived from two years and a half to four years, and have no relapse. From these findings, the author believes that in the opeartion of lung cancer, the preoperative administration of Bleomycin of 150 mg, combined with fibroblast-inhibiting agents, would be beneficial for the prevention of the postoperative recurrence and of the lung fibrosis due to Bleomycin.