Purpose: This prospective study was performed to evaluate the usefulness of preoperative radioimmu-noscintigraphy and intraoperative scintimetric examination (radioimmunoguided surgery: RIGS) using (99m)Tc-anti-CEA F(ab')₂, fragment. Materials and Methods: Nineteen patients with rectal cancer underwent preoperative whole body planar scintigraphy at 4 hours after injection of (99m)Tc-anti-CEA F(ab')₂, fragment and SPECT imaging at 18 hours. Surgical operation was performed at 24 hours after injection. During laparotomy, radioactivities from intraabdominal viscera were measured by gamma probe. The radioac-tivities from excised tumor and lymph nodes were also measured and compared with pathology. Results: All nineteen patients were confirmed to have adenocarcinomas in the rectum. Twenty-seven of 97 excised lymph node groups had metastasis and 2 patients had liver metastasis in pathology. Preoperative radioim- munoscintigraphy detected primary tumors in 11 patients (sensitivity 55% ) and it couId not detect any lymph nodes or liver metastasis. All patients showed high radioactivity in the kidneys, liver, spleen, and major vessels in intraoperative measurement by gamma probe, and tumor activity was not discriminated from background activity. However, ra4ioactivity from excised tumor was higher than normal rectum (T/B ratio; 3.47±2.25). When excised lymph node activity/background activity ratio >1,5 was considered as positive criteria of metastasis, sensitivity, specificity, positive and negative predictive values were 78.6%, 73.9%, 55.0% and 89.5%, respectively. Conclusion: Radioimmunoscintigraphy using (99m)Tc-anti-CEA F(ab')₂. has no additional value for preoperative staging and use of early RIGS using (99m)Tc-anti-CEA F(ab')₂is inappropriate. For early RIGS using (99m)Tc labeled antibodies in rectal cancer patients, further development of more specific antibodies and methods to reduce background activity are needed. (Korean J Nucl Med 2000;34:243-51)