The gene expression profiles of human thyroid carcinomas were analysed by serial analysis of gene expression (SAGE) which allows quantitative and simultaneous analysis of a large number of transcripts. More than 29 000 transcripts derived from a normal thyroid tissue and four thyroid tumours were analysed. While extensive similarity was noted between the expression profiles of the normal thyroid tissue and three differentiated thyroid tumours, many transcripts, such as osteonectin, a-tubulin, glyceraldehyde-3-phosphate dehydrogenase, glutathione peroxidase, and thyroglobulin, were expressed at extremely different levels in differentiated and undifferentiated carcinomas. These data provide new information that might be used to identify genes useful for the diagnosis and treatment of thyroid carcinomas.© 2000 Cancer Research Campaign http://www. bjcancer. com