Purpose: Practically all solid malignant tumours have central hypoxia, which makes them less vulnerable to most forms of both radiotherapy and chemotherapy. Estramustine phosphate (EMP) is known to sensitise cancer cells and tumours to irradiation and to increase blood flow in malignant tumours. This study was designed to assess the effect of EMP and radiotherapy on the oxygenation status of the tumours. Methods and Materials: Nude mice with DU 145 human prostate cancer cell tumours were divided into four groups: The group ER was treated with EMP 0.2 mg/d and external irradiation 3 x 6 Gy; group E received EMP, group R irradiation only; and group O no treatment. The degree of hypoxia in the tumours before and after treatment was measured using 18 F-labelled fluoromisonidazole ([ 18 F]FMISO) as a marker. The testis served as a control organ. Histological samples were studied for necrosis and proliferation (DAPI-stain for mitoses and Ki-67). Results: The tumours showed more [ 18 F]FMISO uptake, indicating more hypoxia, than the testes in all four groups. EMP did not improve tumour oxygenation but enhanced the ability of radiotherapy to cause tumour necrosis. Conclusions: The angiogenic effect of EMP, if present, is not crucial in the mechanism of radiosensitation.