The intraoperative incidence of detectable bilateral and multicentric disease in papillary cancer of the thyroid. This is a prospective study to determine the incidence of grossly detectable multicentric and bilateral cancer at operation in patients who, before surgery, were believed to have a unilateral lesion. The opposite lobe was inspected at the time of operation and the surgeon made a determination as to whether there was bilateral and/or multicentric disease present. A total thyroidectomy was then carried out and the accuracy of the surgeon's judgment was established by permanent-section pathologic examination. Fifteen of the 44 patients were assessed to have gross bilateral disease. Thirteen of these were confirmed by pathologic examination. One case of microscopic bilateral disease was not recognized by the surgeon. Seventeen patients were thought to have unilateral multicentric disease. Fifteen of these cases were confirmed by microscopy and an additional seven cases were documented to have secondary foci. It is obvious from this study that most disease that is called microscopic disease is actually palpable. A review of the literature confirms this. It was very unlikely for a patient who did not have unilateral multicentric disease to have a contralateral focus of carcinoma. The incidence of bilateral disease was 32%, and the incidence of multicentric disease was 50%; the surgeon was very accurate in assessing this. We believe that surgeons who advocate lobectomy as the primary treatment for thyroid cancer are recognizing grossly detectable disease in a significant number of patients and thus are doing total thyroidectomies in most patients with bilateral disease. 