From January 1, 1931 through December 31, 1950, 181 patients with cancer of the thyroid were admitted to the Massachusetts General Hospital. In all cases included in this series, the diagnosis was established by histological examination. In 162 cases, satisfactory follow-up information was obtained. The data on these patients is presented. The cases were classified by standard histological criteria. The great majority (95%) fell into 3 main groups papillary, alveolar cell, and anaplastic carcinoma. The remaining cases consist of tumors of mixed histological characteristics, Hurthle cell and epidermoid carcinomas. Out of the entire group, 48% of the cases died of cancer. A discussion of the data and implications of this study in respect to the management of cancer of the thyroid is presented.