The purpose of this study was to evaluate the usefulness of a radiolabeled analog of somatostatin (iodine-123 octreotide) in the detection and localization of known carcinoid and islet cell tumors and to correlate tumor uptake with the presence or absence of somatostatin receptors. I-123 octreotide studies were performed in 28 patients. Whole-body and tomographic studies were performed over a 2-day period after injection. Twenty-two of the 28 patients underwent tumor biopsy, and samples were analyzed for the presence of somatostatin receptors. Tumors were best seen on scans obtained 1-4 hours after injection. Of the 28 patients, 22 had positive scans with uptake in tumors, three showed photon-deficient uptake in regions of known tumor, and three had negative scans. Seventeen patients in whom results of tumor biopsy were positive for somatostatin receptors had positive scans, and one patient in whom results of biopsy were negative for somatostatin receptors had a negative scan. Previously unsuspected lesions were detected on the I-123 octreotide scans in four of the 28 patients. I-123 octreotide appears to be a useful tracer for the localization of neuroendocrine tumors and, most likely, other soft-tissue tumors as well.