Acquired drug resistance is a major problem in the treatment of cancer. Of the more than 500,000 annual deaths from cancer in the United States’, many follow the development of resistance to chemotherapy. The emergence of resistance depends in part on the genetic instability, heterogeneity and high mutational rate of tumour cells’. In contrast, endothelial cells are genetically stable, homogenous and have a low mutational rate. Therefore, anti-angiogenic therapy directed against a tumour’s endothelial cells should, in principle, induce little or no drug resistance, Endostatin(3), a potent angiogenesis inhibitor, was administered to mice bearing Lewis lung carcinoma, T241 fibrosarcoma or B16F10 melanoma. Treatment was stopped when tumours had regressed. Tumours were then allowed to re-grow and endostatin therapy was resumed. After 6, 4 or 2 treatment cycles, respectively, no tumours recurred after discontinuation of therapy, These experiments show that drug resistance does not develop in three tumour types treated with a potent angiogenesis inhibitor, An unexpected finding is that repeated cycles of antiangiogenic therapy are followed by prolonged tumour dormancy without further therapy.