Background: The importance of changes in tumour-associated stroma for tumour initiation and progression has been established. Endosialin is expressed in fibroblasts and pericytes of blood vessels in several types of tumours, and is involved in the progression of colorectal cancer. In order to see whether endosialin was related to radiotherapy (RT) response, and clinicopathological and biological variables, we investigated endosialin expression in rectal cancers from the patients who participated in a Swedish clinical trial of preoperative RT. Methods: Endosialin was immunohistochemically examined in normal mucosa, including distant (n = 72) and adjacent (n = 112) normal mucosa, and primary tumours (n = 135). Seventy-three of 135 patients received surgery alone and 62 received additional preoperative RT. Results: Endosialin expression in the stroma increased from normal mucosa to tumour (p andlt; 0.0001) both in RT and non-RT group. In the RT group, endosialin expression in the stroma was positively associated with expression of cyclooxygenase-2 (Cox-2) (p = 0.03), p73 (p = 0.01) and phosphates of regenerating liver (PRL) (p = 0.002). Endosialin expression in the tumour cells of both in the RT group (p = 0.01) and the non-RT group (p = 0.06) was observed more often in tumours with an infiltrative growth pattern than in tumours with an expansive growth pattern. In the RT group, endosialin expression in tumour cells was positively related to PRL expression (p = 0.02), whereas in the non-RT group, endosialin expression in tumour cells was positively related to p73 expression (p = 0.01). Conclusions: Endosialin expression may be involved in the progression of rectal cancers, and was related to Cox-2, p73 and PRL expression. However, a direct relationship between endosialin expression and RT responses in patients was not found.