Capsule invasion observed in instances of adenomas of the thyroid was studied in correlation with the clinical course of the patients. The invasion, as defined in the cut-off and turn-out of the collagen fibers in the capsule, occurred in eight of 611 adenomas. The clinical course of the eight patients revealed no recurrence of an adenoma, even though there was no antitumor drug therapy. On the other hand, recurrences of adenomas were found in five of the 603 patients with noninvasive adenoma, of the 603 patients with noninvasive adenoma. Capsule invasion, therefore, is not a cause for the recurrence of an adenoma. The recurrence may result from the leaving of microadenoma in the thyroid when the first operation took place. Capsule invasion cannot be considered as a histologic sign of malignant tumor of the thyroid.