The US Preventive Services Task Force (USPSTF) has given thyroid cancer screening a D grade, recommending against screening. 1 They conclude with moderate certainty that screening for thyroid cancer in asymptomatic persons results in harms that outweigh the benefits. Given the historical contextboth in pathology and medical care epidemiology-it should have been an easy decision.At the end of World War II, there was an alarming publication by a group of Chicago surgeons reporting that 17% of nontoxic thyroid nodules harbored thyroid cancer. The surgeons concluded that this finding makes "surgical therapy quite urgent in this seemingly harmless lesion." 2(p883) A few months later, New York surgeons reported that apparently benign thyroid nodules and innocent-appearing breast lumps had a similar prevalence of cancer. 3 They concluded that it is as justifiable to perform a thyroidectomy for a thyroid nodule as a biopsy for a breast mass.In 1947, a Boston pathologist responded to these high prevalence estimates by raising questions about the criteria for thyroid cancer and how they might differ among pathologists. He argued that "the ultimate criterion for the diagnosis of cancer is that the disease naturally results in the death of the patient harboring it." 4(p221) And he went on to emphasize that thyroid cancer was an extremely rare cause of death.The extent of the high prevalence/rare death mismatch was made even more evident in 1985. Finnish pathologists reported on their work systematically sectioning the thyroid gland in 101 consecutive autopsies. 5 They found that 36% contained papillary thyroid cancer, the majority of which were very small. Given the number of small cancers they found and the number that they reasoned they missed, the researchers deduced that virtually everybody would have some evidence of thyroid cancer if examined carefully enough. 6 They concluded that small papillary thyroid cancers are "so common in Finland, possibly also in other countries, that they can be regarded as a normal finding." 5(p538) Theoretically, at least, there was a bottomless reservoir of undetected thyroid cancer.But there was no epidemiologic evidence that medical interventions were beginning to tap this reservoir until the mid 1990s. SEER data 7,8 show that thyroid cancer incidence had been relatively stable in the United States-but has since shot up 3-fold (Figure). All of the increase is due to papillary thyroid cancer. And despite the dramatic increase in incidence, mortality from thyroid cancer has remained stable-a combination that is pathognomonic for overdiagnosis.While it was clear what was happening, it was less clear why it was happening. 9 Some of the increase undoubtedly reflected incidental detection-serendipitous detection of thyroid can-