Objective To assess the diagnostic value of spiral CT colonography(SCTC)with orally administered Mannitol as negative contrast in colorectal tumor.Methods 27 volunteers and 68 patients suspected of various kinds of colorectal diseases were examined.About 1500 ml isosmotic Mannitol as oral negative contrast agent was administered and then 20mg of Raceanisodamine hydrochloride was injected intramuscularly,three phase enhanced spiral CT scanning was performed 5 to 10 minutes later.imaging data were post-processed with multiplanar reformation(MPR),maximum intensity projection(MIP).All normal colorectal were correctly subarea and were measured intestinal wall thickness and maximum outer diameters,the CT values of bowel wall in different phases were measured either,the bowel with tumor as control,and the results were statistically calculated.SCTC was classifield into three Grades according to the effect of intestinal preparation.CT data of 61 cases colorectal tumors identified by pathology were retrospective analysed.Results SCTC was successfully done in all objects,the SCTC grading included Grade 1 in 52 cases,Grade 2 in 39 cases and Grade 3 in 4 cases.The total rate of satisfaction towards examinantion was 95.8%.The lesions in 60/61 cases were demonstrated clearly by SCTC,among them,there were carcinoma in 30,lyphoma in 8,abscess in 8,polyps in 8,metastases in 4 and carcinoid in 3.One case with polyp(less than 5 mm)was missed.The sensitivity and specificity of SCTC in diagnosing colorectal lesions were 98.36% and 78.9% respectively.27 cases with colorectal carcinoma were correctly staged in Duke and the accuracy was 90%.Conclusion SCTC is a simple,rapid,noninvasive and effective method for evaluating colorectal tumor.