Computed tomography is well known modality for detecting of rectal anal cancer. Nineteen cases of anal and rectal cancer were analyzed about perirectal invasion, metastasis, and definded their staging by CT. RESULTS : 1. The CT scans demonstrated primary cancer of anus and rectum in all cases, and correctly staged in 14 patients. 2. The CT showed an accuyracy for detecting lymphnode involvement of 68%, and perirectal invasion of 89%. 3. The perirectal invasion suggests a metastasis to lymphnode or other organs.