This paper presents a patient who was operated on for preoperative diagnosis of lymphangiosis type of primary diffusely infiltrating carcinoma of hte colon and rectum but experienced metastasis to the brain one month after the operation, with a fatal course. A 47-year-old man complaining of severe diarrhea was suspected of having inflammatory intestinal disease such as idiopathic ulcerative colitis, but was demonstrated having an denocarcinoma on biopsy at another hospital. The patient was referred to the center with a probable diagnosis of metastatic colorectal cancer. Barium enema examination demonstrated an irregularity of the intestinal wall, and difficulties of extension almost 30cm in length at the signoid portion and the rectum level. But no stenosis was found. Endoscopically, the mucosa was intact but edematous and rough. There was no luminal mass and only the small, shallow ulcerlation was observed. The patient underwent abdominoperineal resection of the tumor at the sigmoid colon and rectum, resulting in non-curative resection because of extensive abdominopelvic lymph nodes metastasis including involvement of the para-aortic lymph nodes. No evidence of tumor involvement in any other abdominal or pelvic organs was evident at the time of surgery. The surgical specimen revealed a diffusely thickened and irregular wall with a 2cm shallow mucosal ulcer. Histologically, the tumor was poorly differentiated adenocarcinoma except the small area of moderately differentiated adenocarcinoma under the ulcer. The patient received a course of chemotherapy (CPT-11) postoperatively, but after a week, the brain metastasis caused brain edema and his consciousness was getting down. The patient's clinical course continued to deteriorate due to the progression of the cancer, a nd he died two months after the operation. Primary diffusely inflitrating carcinoma of the colon is a relatively rare primary colorectal malignancy. But the prognosis is poor and its clinical significance is large. The poor prognosis is likely related to the initial delay in its diagnosis especially in the lymphangiosis type, because its symptom is sometimes unclear and the lesion is often similar to inflammatory colonic lesions radiographically and microscopically.