The spectrum of p53 gene mutations was investigated in thyroid carcinomas with respect to histopathological classification. In all histological subtypes of thyroid carcinoma that had previously revealed positivity in immunohistochemical staining for p53 protein, single-stranded conformation polymorphism analysis and direct sequencing were performed to detect point mutations between exons 5 and 8. In well differentiated papillary and follicular carcinomas, in which we had already known that 11.1 and 14.3% of the cases, respectively, revealed p53 overexpression as determined by immunohistochemistry, genetic aberrations were undetectable. In poorly differentiated carcinoma, in which 40.9% had revealed overexpression, two of six cases revealed point mutations at codon 244 in exon 7 and at codon 278 in exon 8. In undifferentiated carcinoma, in which 63.6% had revealed overexpression, four of six cases examined showed point mutations at codon 157 in exon 5, at codon 248 in exon 7, and at codon 273 and a two-base insertion between codons 266 and 267 in exon 8. These results strongly suggest the crucial role of p53 gene aberration and protein overexpression in a biologically aggressive subtype, possibly as a stepwise participation in the process of tumor dedifferentiation in human thyroid carcinomas.