Although elevated c-myc expression seems to be related to an unfavorable prognosis of human thyroid neoplasias, the role of c-myc overexpression in the process of thyroid carcinogenesis is still unknown. We analyzed c-myc expression in 7 human thyroid carcinoma cell lines, originating from different histotypes, and in 50 fresh thyroid tumors and found a higher level of c-myc mRNA in all the thyroid carcinoma cell lines and in several fresh thyroid tumors compared with normal thyroid. The highest increases occurred in the most malignant cell lines and in undifferentiated human thyroid carcinomas. The block of c-MYC protein synthesis with myc-specific antisense oligonucleotides reduced the growth rate of the thyroid carcinoma cell lines significantly. Our results indicate that c-myc overexpression plays a critical role in the growth of thyroid cancer cells, which supports the hypothesis that the myc proto-oncogene might be involved in the neoplastic progression of thyroid carcinogenesis.