Current models for tumorigenesis propose that a series of genetic alterations occur during the progression from the normal cell to the malignant phenotype. Mutations in each of the three ras genes (K-ras, H-ras, and N-ras) have been identified in many human neoplasms, including thyroid cancer. In this study we examined genomic DNA from benign and malignant thyroid neoplasms for mutations that are known to activate the ras oncogenes (codons 12, 13, and 61). DNA from frozen surgically excised tissue (n = 8) and from formalin-fixed paraffin-embedded tissue (n = 30) was amplified by the polymerase chain reaction and screened for mutations using oligonucleotide-specific hybridization. No mutations were identified in follicular adenomas (n = 9). In follicular carcinomas, 2 of 14 tumors contained mutations (N-ras 61, Gln to Arg), and both of these patients had bone metastases. One of 15 papillary carcinomas had a ras mutation (H-ras 12, Gly to Ser). In contrast to other studies, we found that ras mutations are relatively uncommon in both benign and malignant thyroid neoplasms. Studies of larger numbers of tumors and comparisons of different patient populations will be required to assess a possible association of mutations in N-ras 61 with clinically aggressive follicular cancer.