To evaluate the importance of fecal leukocytes, 42 patients who showed signs of fecal leukocytes (++ or +++) were studied. Their endoscopic examinations with biopsy and/or radiology of the colon showed the following diagnoses: 33 had ulcerative colitis, four had colonic adenocarcinoma, two had Crohn's disease, two had amebic colitis and one had eosinophilic colitis. The presence of fecal leukocytes allowed for the diagnosis of colon disease in all the patients, and it might indicate exudative bowel disease. These results suggest that whenever fecal leukocytes are found in the feces, an examination for colon disease should be made.