Not all bowel cancers have symptoms of occult blood. Occult blood refers to a small amount of bleeding that does not cause change in stool color and requires occult blood test to determine, which is called occult blood. Generally speaking, a positive occult blood test, i.e. occult blood, can be seen when gastrointestinal bleeding is within 5-10 ml per day. Not all bowel cancers have occult blood. Bowel cancer is most common in colon cancer and rectal cancer. The earliest symptom of colon cancer is the change of bowel habit and fecal character, which mostly manifests as increased frequency of bowel movement, diarrhea, constipation, blood, pus or mucus in feces. Meanwhile, abdominal pain, abdominal mass, intestinal obstruction and systemic symptoms can also appear. Generally, right half colon cancer is mainly characterized by abdominal pain, abdominal mass and systemic symptoms, while left half colon cancer is mainly characterized by obstruction, change of defecation habit and nature of feces. The probability of having blood in stool in rectal cancer is 80% to 90%. Early stage of rectal cancer has no obvious symptoms, and symptoms appear only after the cancer affects defecation or bleeds after rupture. Bleeding from cancer rupture can be manifested as blood and mucus in stool, or even pus and blood in stool. Besides, there may be rectal irritation sign, intestinal stenosis and other manifestations. Meanwhile, occult blood can also be seen in many diseases, such as intestinal tuberculosis, ulcerative colitis, dysentery, hemangioma of small intestine, loss of rectal-anal tube, and bleeding of peptic ulcer and so on. As bowel cancer has various manifestations, not all bowel cancers show occult blood manifestations, while occult blood can also be seen in many other diseases. In order to avoid the development of the disease and delay of treatment, it is necessary to consult a doctor in time, improve the detailed examination under the advice of a specialized doctor to clarify the cause of the disease and take early treatment.