A series of 20 cases of radiation bowel disease (RBD) was studied qualitatively and the arterial changes were studied quantitatively. A control series of 45 cases was studied. In the control cases there were positive correlations between the medial thickness of all vessels studied and the diastolic blood pressure as well as the incidence of intimal fibrosis in both intramural and extramural arteries. The medial thickness in all the arteries in cases of RBD was significantly higher than in the controls. This was probably due to the large number of fibrin thrombi which increased the vascular resistance. The degree of intimal fibrosis of the intramural arteries and arterioles was significantly greater than in the controls. Similarly the incidence of intimal fibrosis in all arterioles and intramural arteries was greater than the control group. The degree of intimal fibrosis was related to the dose of radiation received. The effect of radiation was an on-going process since the percentage of arterioles with intimal fibrosis increased with the time after radiotherapy. Blood pressure and age played no part in these correlations in RBD. The most consistently observed qualitative changes in RBD were in the arteries, arterioles and to a lesser extent the veins. These showed fibrin thrombi, fibrinoid necrosis, subendothelial oedema and fibrin. Various stages of healing were seen in the vessels. We believe that the blood vessels are the main site of injury in RBD and that the endothelial cell is the initial target for radiation damage.