Hyperfunctioning thyroid adenomas causing thyrotoxicosis (Plummer's disease) is not an uncommon cause of hyperthyroidism in the elderly. Most commonly, the adenoma appears as a so-called "hot" nodule on thyroid scintigraphy causing suppression (i.e., nonvisualization) of the remainder of the gland. This report describes a case of Plummer's disease in an elderly patient in whom the toxic nodule primarily responsible for causing the hyperthyroidism became scintigraphically apparent at 96 hours after I-131 therapeutic ablation.