We used positron emission tomography (PET) with [18F]fluoromisonidazole ([18F]FMISO) to study tumor hypoxia in six dogs with spontaneous sarcomas. The tumors were regarded as hypoxic if [18F]FMISO uptake exceeded normal tissue radioactivity by 40% (tumor/muscle ratio > 1.4) or if kinetic analysis indicated a positive [18F]FMISO tissue influx rate (Ki > 0) by a Patlak plot. Using these criteria, we found hypoxia in a fibrosarcoma grade II, an undifferentiated sarcoma, and an ostoeosarcoma, but not in a fibrosarcoma grade I, another osteosarcoma, and a myxosarcoma. In three animals, the tumor oxygen partial pressure (pO2) was also measured invasively using Eppendorf needle electrodes. In these cases, the Eppendorf measurements were confirmed by the [18F]FMISO PET results. In addition, [15O]H2O PET was performed in four dogs in order to assess tumor perfusion. Comparisons of the [18F]FMISO with [15O]H2O PET images in two cases showed that tumor hypoxia occurred in the tumor center with low perfusion, whereas perfusion was heterogeneous in a nonhypoxic tumor.