Tumor hypoxia negatively regulates cell growth and causes a more malignant phenotype by increasing the expression of genes encoding angiogenic, metabolic and metastatic factors. Of clinical importance, insufficient tumor oxygenation affects the efficiency of chemotherapy and radiotherapy by poorly understood mechanisms. The hypoxia-inducible factor (HIF)-1 is a master transcriptional activator of oxygen-regulated genes and HIF-1 is constitutively upregulated in several tumor types. HIF-1 might thus be implicated in tumor therapy resistance. We found that transformed mouse embryonic fibroblasts deficient for HIF-1α are more susceptible to the treatment with carboplatin, etoposide and ionizing radiation than wild-type cells. Increased cell death in HIF-1α-deficient cells was because of apoptosis and did not involve p53 induction. Tumor chemotherapy of experimental fibrosarcoma in immunocompromised mice with carboplatin and etoposide confirmed the enhanced susceptibility of HIF-1α-deficient cells. Agents that did not cause DNA double-strand breaks, such as DNA-synthesis inhibitors or a DNA single-strand break-causing agent equally impaired cell growth, independent of the HIF-1α genotype. Functional repair of a fragmented reporter gene was decreased in HIF-1α-deficient cells. Thus, hypoxia-independent basal HIF-1α expression in tumor cells, as known from untransformed embryonic stem cells, is sufficient to induce target gene expression, probably including DNA double-strand break repair enzymes.