The exact risk of multiple primary neoplasms in patients with thyroid cancer is difficult to ascertain from the data available in the literature. Three thousand seventy-two patients with thyroid cancer, listed in the Israel Cancer Registry during a 16-year time span, were studied to determine the true incidence of another primary cancer. Ninety-two cases were reported as having an additional primary cancer. The prevalence of multiple primary malignancies was 3%. The frequency was higher among patients of European rather than of Asian or African origin. The second primary cancers in order of decreasing frequency were of the breast, lung, colorectum, head and neck, and lymphoma/myeloma. Most of the deaths were due to the additional cancer. The 5-year survival rate was highest for head and neck and lowest for lung cancer patients. These results emphasize the need for greater awareness of the possibility of developing additional cancers, and indicate the need to incorporate strategies for the prevention, early detection, and treatment of multiple primary neoplasms.