Scintillation scanning of the clinically single thyroid nodule before surgery can aid the preoperative detection of malignancy and other processes not likely to respond to medical therapy. Preoperative scintiscanning with radioactive iodine (I131) was carried out in 103 patients, each with a clinically single thyroid nodule, who came to surgery. On this basis, the nodule was classified as non-functional in 24 cases, and the incidence of carcinoma (14 cases, 58%) was highest in this group. It was lowest (2 cases, 5%) among the 43 patients with nodules classified as hypofunctional. This diagnostic aid makes it possible to limit treatment to medical therapy in at least half of patients with single thyroid nodules, with the expectation that surgery can be safely avoided in one quarter of the entire group.