More than 300 new cases of thyroid cancer are diagnosed in Los Angeles County every year. The age-adjusted annual incidence rates of this disease for all races combined are 2.4 for males and 6.1 for females. Rates for women are more than twice rates for men in each major ethnic group. Blacks of both sexes have the lowest rates; Japanese, Chinese, other Asians and Spanish-surnamed whites all have rates that are as high as or higher than rates among non-Spanish-surnamed whites. Other demographic patterns include the excess of thyroid cancer among Jewish residents of Los Angeles. There have been an increase in thyroid cancer incidence and a decline in mortality for this disease in the United States over the past several decades. Several possible explanations can be made for these trends. Also, the risk factors for thyroid cancer deserve review.