Serum TSH levels, determined by a very sensitive immunofluorescent assay, were found to be consistently higher in euthyroid individuals with thyroid cancer than in normal persons or those with other thyroid diseases or with non-thyroidal cancers. Thyroid cancer patients required larger thyroxin doses for TSH suppression than controls. From these studies it appears that chronic TSH stimulation, long known to produce thyroid cancer in experimental animals, may also play a causative role in human thyroid cancer.