AIMS AND BACKGROUND: Intraoperative localization, during open and laparoscopic surgery, of small, nonpalpable colonic lesions located at peculiar sites or with concurrent inflammatory bowel alterations (diverticulosis, perivisceritis) is often difficult. The aim of our work was to assess the validity of radioguided identification after preoperative labeling. METHODS AND STUDY DESIGN: Patients who were candidates for colon surgery for occult lesions that, because of their size and location, were assumed to be difficult to detect, underwent colonoscopy 1 to 2.5 hours before surgery. A small dose of labeled albumin macroaggregates was injected with a sclerotherapy needle into the subserosa underneath the lesion. Immediately following the injection the lesion was identified with a transcutaneously placed gamma detecting probe. Intraoperative tracer detection was performed either during open surgery or by means of a laparoscopic probe (detection time 3-5 mins). The position of the lesion was marked with a suture or with a clip. Surgery was performed according to the type of lesion to be treated. RESULTS: In our initial clinical experience 15 colon lesions were preoperatively marked in 14 patients and were subsequently detected during surgery (four under laparoscopy) with a gamma detecting probe. This technique allows highly accurate, fast, and inexpensive surgical localization of lesions without irradiation and without complications. CONCLUSION: Our experience shows that preoperative endoscopic marking of nonpalpable colon lesions with 99mTc-labeled albumin macroaggregates followed by intraoperative detection with a gamma probe is a useful clinical method that is highly accurate and without complications.