To assess the morbidity and mortality of thyroid operations, Professional Activity Study records of the Commission on Professional and Hospital Activities, representing an estimated one-third of all thyroidectomies performed in the United States in 1970, were reviewed. The mortality after a thyroid operation for nontoxic goiter was 0.02 per cent for patients less than the age of 50 years but increased with age to 0.66 per cent for those 70 years and older. No in-hospital deaths followed thyroidectomy for malignant goiter in 766 patients less than 40 years of age. Thyroidectomy for diffuse toxic goiter had a mortality fivefold greater than did operations for a benign nontoxic goiter. Total thyroidectomy was used for the treatment of nontoxic, nonmalignant goiter for one in 12 patients and resulted in greater morbidity than did partial or subtotal thyroidectomy. Rational choice of operation for the treatment of goiter for individual patients should be based upon knowledge of the specific risks of surgical treatment, which vary greatly depending upon the age of the patient, whether or not the goiter is toxic or nontoxic and whether or not subtotal or total thyroidectomy is performed.