Using the method of polymerase chain reaction‐single strand conformation polymorphism, the point mutations of the ras oncogenes in a total of 33 thyroid tissues, including 12 follicular adenomas, 6 follicular carcinomas, 11 papillary carcinomas, and 4 undifferentiated carcinomas, were examined. The frequency of the mutation was 3 (1/33) in codon 12,13 of Ki‐ras and 18 (6/33) in codon 61 of N‐ras, including 17 (2/12) in follicular adenoma, 50 (3/6) in follicular carcinoma, 0 (0/11) in papillary carcinoma and 25 (114) in undifferentiated carcinoma. In follicular adenoma, positivity was observed in microfollicular or trabecular subtypes. Furthermore, the mutation of ras was examined in histologically different parts, coexisting in the same tumor in a total of four cases. Both the undifferentiated carcinoma and coexisting follicular adenoma, and both the microfollicular adenoma and trabecular nodule growing in the tumor, had the same N‐ras (61) mutation. Direct sequencing analysis showed that all mutations were CAA (Gln) to CGA (Arg) transition of codon 61, except for CAA to AAA transversion in one case of follicular carcinoma. A similar genetic abnormality of N‐ras genes at codon 61 between follicular adenoma and follicular carcinoma suggests that the mutation of N‐rasat codon 61 might play a part in oncogenesis in follic
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