Previous studies have demonstrated that point mutations in all three ras genes (H-ras, K-ras, and N-ras) may occur in thyroid neoplasia. However, the overall incidence of ras mutations in thyroid tumors and their frequency in specific histologic types varies widely in different series. Many earlier studies have chosen allele-specific oligonucleotide hybridization approaches to examine ras mutations without further confirmation of the positive samples by DNA sequencing. In this study, mutational hot spots in exon 1 (codons 12/13) and exon 2 (codon 61) of the H-ras, K-ras, and N-ras were polymerase chain reaction (PCR) amplified and sequenced with an automatic sequencer. ras mutations were detected in 4 of 89 (4.5%) benign and malignant thyroid tumors. Three of 8 follicular carcinomas exhibited mutations in codon 61 of H-ras, K-ras, and N-ras, respectively, and mutation at codon 61 of N-ras was found in 1 of 12 follicular adenomas. No mutations were observed in the other tumors, which included 20 nodular goiters, 5 Hürthle cell adenomas, 42 papillary carcinomas, and 2 undifferentiated carcinomas. Our results, obtained by the direct sequencing technique, indicate a lower overall prevalence of ras oncogenes in thyroid tumors than reports in earlier series. However, the frequency of ras mutations in specific histotype of thyroid tumors and their exclusive involvement of codon 61 in our series are similar to those studies utilizing DNA sequencing to detect or to confirm ras gene alterations. The selective occurrence of ras mutations in benign and malignant follicular neoplasms indicates that ras gene alterations have a specific and early role in the development of follicular type of thyroid tumors in Taiwan.