For successful tissue engineering in surgical radiotherapy patients, irradiated endothelial cells (EC) must form new blood vessels to nourish and build connections with the engineered segment. Therefore, it is critical to understand neovasculogenesis by irradiated EC. The objective of this study was to determine the effects of ionizing radiation on endothelial cell proliferation and capillary-like structures (CLS) formation. Human Umbilical Vein Endothelial Cells (HUVEC) were irradiated with single or fractionated doses of radiation. Proliferation was determined by counting cells. CLS morphology was analyzed from photomicrographs. A single dose of 8 Gy radiation was highly lethal to HUVEC compared to lower dosage. A single dose had more of an inhibitory effect on cell proliferation compared to the same dose delivered in a fractionated manner. CLS formation began after cells reached confluency. To form a CLS, a single cell expanded, and a number of cells rearranged around its periphery in an oval fashion (mimicking a vessel wall). The central cell later disintegrated leaving a void, mimicking the lumen. Irradiated EC can form CLS, although they are fewer and smaller compared to those by sham cells. By disrupting the peripheral cells, >or=4 Gy doses significantly reduced the number of CLS. The disruptive affect was seen more with large CLS compared to small CLS. At different doses, the shapes of CLS were not significantly different.