Tc-99m pertechnetate and I-123 were used to perform thyroid scanning in 122 patients with history or clinical evidence of thyroid disease. Thyroid scans were abnormal in all patients, while thyroid palpation was abnormal in all but 19. The quality of thyroid imaging was similar with both agents in 42%, better with I-123 in 18%, and better with Tc-99m in 7%. In the remaining 33% (40 cases), there were discrepancies between Tc-99m and I-123 images. The most frequent discrepancies were "hot" or "warm" lesions on Tc-99m scans that were "cold" or normal on I-123 scans. Results from this study indicate that neither Tc-99m nor I-123 is always superior to the other as a thyroid imaging agent.