BACKGROUND AND PURPOSE: The level of hypoxia in primary tumors has been described to influence response to treatment. The aim of the present study was to investigate the impact of pretreatment oxygen level measurements in spontaneous canine tumors on treatment outcome. MATERIALS AND METHODS: Data of pretreatment tumor oxygenation status and local tumor response after primary radiation therapy in a group of spontaneously occurring tumors in dogs (n=52) was collected. Radiation therapy was given with curative (14-17x3-3.5 Gy) or palliative intent (3x8 Gy or 4-5x6 Gy). Progression-free interval and overall survival were correlated to polarographically measured tumor oxygenation status. RESULTS: In the curatively irradiated group, tumors with median pO2 values<or=10 mmHg tended to have shorter median progression- free interval compared to better oxygenated tumors (246 vs. 739 days). The same trend could be shown for overall survival (330 vs. 745 days), indicating a cutoff value in this region. In the group treated with lower doses of radiation, the level of oxygen was no longer found to be of prognostic value; however, in this group hemoglobin had a significant impact on outcome. CONCLUSION: In curatively irradiated spontaneous canine tumors, tumor hypoxia was found to be a prognostic indicator, independent of tumor histologies and volume.