Objective To evaluate the diagnostic accuracy of CT (computed tomography) in detecting long bone nonunion of extremities. Methods We collected the clinical data of 118 patients with long bone nonunion or equivocal long bone nonunion of extremities who had been admitted to our department from February 2003 to May 2007.Of them, 33 patients with equivocal findings on plain radiographs underwent CT scanning. Union or nonunion was determined according to either intraoperative findings (in 114 patients who had been operatively treated) or clinical observation of more than 6 months (in 4 patients who had been diagnosed as union by CT).Their CT scans and plain radiographs were assessed by experienced orthopedists in double blind manner. The sensitivity and specificity of CT scan were evaluated by the receiver operating characteristic curve. Results CT scans displayed very good diagnostic accuracy. The sensitivity of CT scads for detecting nonunion Was 100%. And its overall accuracy Was 83,3%.The specificity of CT scans for detecting nonunion was 66.7%.for 2 patients who had been diagnosed as nonunion by CT were found in surgery to have a partial union. Intraobserver agreement Was high (interelass correlation coefficient=0.91). Conclusion CT scans have a very good diagnostic accuracy, when a plain radiograph can not determine union or nonunion of a long bone fracture of extremity.