Radioimmunodetection (RAID) using Tc-99m labeled Fab' fragments of the anti-CEA monoclonal antibody IMMU-4 was performed in a 68-year-old man for preoperative evaluation of colonic metastases. Planar images at 3 and 18 hours showed uptake within known metastases in the left lobe of the liver that had been observed with CT and at surgery during the preceding month. SPECT images at 3 hours demonstrated two additional unsuspected lesions. All sites were surgically confirmed the following day using intraoperative sonographic guidance, with the smallest occult lesion measuring only 6 x 9 mm. This case illustrates the limitations of CT and surgery, the potential of RAID for the detection of occult metastases, the increased sensitivity of SPECT over planar images for detecting lesions less than 1 cm in diameter, and the value of intraoperative guidance for small nonpalpable lesions.