Protooncogene Ha-ras codon 12 mutations are frequently observed in thyroid cancers. However, their role in the initiation and development of this pathology remains to be clarified. Here we present a preliminary study using 60 samples corresponding to different types of cancer. DNA amplification by PCR (polymerase chain reaction) followed by RFLP (restriction fragment length polymorphism) analysis enabled the detection of a point mutation in codon 12 of the Ha-ras oncogene in human thyroid adenomas and carcinomas. Our results confirm the high frequency of codon 12 Ha-ras oncogene mutation in thyroid tumors: adenomas 33%, with a particularly high rate for atypical adenomas 71%, follicular carcinomas 33%, and papillary carcinomas 19% (n = 18, 7, 12, 26, respectively). No mutation was detected in undifferentiated carcinomas (n = 4). The Ha-ras codon 12 gene point mutation can exist at all stages of development of both benign and malignant thyroid tumors. It may be a necessary part of the thyroid tumorigenesis process, but it is not the only carcinogenic factor. Additionally, the association with other molecular anomalies should be sought depending on the thyroid cancer type.