Inferior mesenteric arteriography was performed prior to surgery on 36 patients with carcinoma of the rectum in order to diagnose the degree of cancer invasion. Results were divided into four categories based on degree of invasion as observed in angiographs, and were compared with results of macroscopic observation and histological studies. A high degree of correlation was seen between observation of pre-operative angiographic abnormalities and macroscopic observation of the degree of cancer invasion (75%). However, comparisons between histological findings and observations of pre-operative angiographic abnormalities yielded a low degree of correlation (41.2%), particularly among the lower three categories of invasion. This is believed to reflect the changes in tissue that accompany cancer, such as inflammation, adhesion, and fibrosis, which also are revealed by angiography. The ability to diagnose the degree of cancer invasion is important in selecting the appropriate surgical procedure for treating rectal cancer, and angiography should be considered for use in such diagnosis. This study showed that pre-operative angiography clearly highlighted changes in rectal arteries in advanced cases (A2, a2 or above) of rectal cancer. The usefulness of angiographic observations in selecting treatment strategies was demonstrated.