Recent data suggest that recombinant human erythropoietin (rhEPO) modulates tumour growth and therapy response. The purpose of the present study was to examine the modulation of radiotherapy (RT) effects on tumour microvessels by rhEPO in a rat colorectal cancer model. Before and after 5 Â 5 Gy of RT, dynamic contrast-enhanced -magnetic resonance imaging was performed and endothelial permeability surface product (PS), plasma flow (F), and blood volume (V) were modelled. Imaging was combined with pO 2 measurements, analysis of microvessel density, microvessel diameter, microvessel fractal dimension, and expression of vascular endothelial growth factor (VEGF), hypoxia-inducible factor-1 a (HIF-1a), Bax, and Bcl-2. We found that RT significantly reduced PS and V in control rats, but not in rhEPO-treated rats, whereas F was unaffected by RT. Oxygenation was significantly better in rhEPOtreated animals, and RT induced a heterogeneous reoxygenation in both groups. Microvessel diameter was significantly larger in rhEPO animals, whereas VEGF expression was significantly lower in the rhEPO group. No differences were observed in HIF-1a, Bax, or Bcl-2 expression. We conclude that rhEPO results in spatially heterogeneous modulation of RT effects on tumour microvessels. Direct effects of rhEPO on neoplastic endothelium are likely to explain these findings in addition to indirect effects induced by increased oxygenation. British Journal of Cancer (2007) Anaemia commonly occurs in colorectal cancer patients especially if they are treated with neoadjuvant radiotherapy (RT) or chemotherapy. Anaemia not only adversely affects the clinical condition of these patients, but also contributes to the development of tumour hypoxia, recognised as a major negative determinant of sensitivity to RT, chemoradiotherapy, and certain chemotherapeutic agents (Harrison and Blackwell, 2004;van Halteren et al, 2004).Recent clinical studies have shown that administration of recombinant human erythropoietin (rhEPO, epoetin alfa) increases haemoglobin levels and improves quality of life in patients with cancer-related anaemia (Littlewood et al, 2001). Over the last decade, it has become clear that the action or rhEPO extends into a wide range of cellular mechanisms involved in stem cell development, maintenance of cellular integrity, and physiological angiogenesis (Maiese et al, 2005). The demonstration of the erythropoietin (EPO) receptor in various neoplastic tissues and the observation in a recent clinical trial that mortality was higher in nonanaemic rhEPO-treated breast cancer patients highlighted the possible effects of rhEPO on tumour growth and angiogenesis (Leyland-Jones, 2003). Preclinical studies investigating the role of EPO and EPO -EPO receptor signalling on tumour growth and angiogenesis have yielded contradictory results. Yasuda et al (2003) noted the inhibition of angiogenesis and tumour cell survival in stomach and melanoma xenografts following blockade of EPO signalling. The results of Hardee et al (2005), however, suggested that a...