PURPOSE: ras oncogenic activation has long been demonstrated in thyroid carcinomas of follicular cell derivation, but no consistent relationship has been shown between mutations and clinicopathologic features. MATERIALS AND METHODS: We analyzed H-, K-, and N-ras mutations by polymerase chain reaction-single-strand conformational polymorphism followed by DNA sequencing in 125 thyroid carcinoma specimens from 107 patients, to include tumors covering the entire spectrum of thyroid tumor differentiation. RESULTS: Mutations were identified in four (8.2%) of 49 well-differentiated carcinomas (WDCs; two [6.7%] of 30 of the tumors were papillary carcinomas, two [10.5%] of 19 of them were follicular carcinomas), in 16 (55.2%) of 29 poorly differentiated carcinomas (PDCs), and in 15 (51.7%) of 29 undifferentiated carcinomas, with a significant association between ras mutation and poorly or undifferentiated tumors (P <.001). Twenty-six (74.3%) of 35 patients with ras-mutated tumors died as a result of disease as opposed to 23 (31.9%) of 72 patients with tumors lacking the mutations. Among patients with differentiated thyroid carcinomas (WDC and PDC), 11 (55.0%) of 20 patients with mutated tumors died as a result of disease as opposed to nine (15.5%) of 58 patients with wild-type ras tumors, and the correlation was independent of tumor differentiation and stage (P =.016). K-ras codon 13 mutations (all with G-A nucleotide transitions resulting in Gly>Asp substitution) and single activating mutations in any of the ras genes were also independent predictors of poor survival in differentiated thyroid carcinomas (P =.027 and P =.007, respectively). CONCLUSION: These findings demonstrate that ras mutations are a marker for aggressive cancer behavior and indicate a possible role of ras genotyping to identify thyroid carcinoma subsets associated with poor prognosis.