Thyroid carcinoma usually is characterized by a nonfunctioning or "cold" nodule depicted on the Tc-99m scan. Twelve patients with functioning "hot" thyroid nodules underwent thyroidectomy (nine hemithyroidectomies, two nodulectomies, and one subtotal thyroidectomy). Histologic examination revealed that five patients (42%) had malignant nodules (papillary carcinoma in two and follicular carcinoma in three). Of the seven patients with benign nodules, five had follicular adenoma and two had adenomatous hyperplasia. TI-201 scanning (early and delayed) was performed at the same time as Tc-99m scanning. Only one of the seven benign nodules showed accumulation on the delayed TI-201 scan, whereas all five malignant nodules showed accumulation. The delayed TI-201 scan is useful for the differentiation of benign from malignant nodules that show accumulation of Tc-99m pertechnetate.