Recently, a putative tumor suppressor gene, PTEN/MMAC1, has been identified at chromosome 10q23.3. This gene encodes a 403 amino acid dual specificity phosphatase containing a region of homology to tensin and auxillin. Somatic mutations of the PTEN/MMAC1 gene have been found in a number of cancer cell lines and primary cancers. Cowden disease, an autosomal dominant harmartoma syndrome associated with thyroid and breast tumors, has been found to be associated with mutations of PTEN/MMAC1 gene. To evaluate the role of the PTEN/MMAC1 gene in sporadic thyroid tumors, we studied 17 sporadic thyroid tumors, of which 12 were papillary thyroid carcinomas, 1 was follicular thyroid carcinoma, 1 was medullary thyroid carcinoma and 3 were thyroid adenomas. Direct sequencing of PCR-amplified products was performed for all nine exons of PTEN/MMAC1. No mutations of PTEN/MMAC1 gene were observed in any of the sporadic thyroid tumors. Our results indicate that the PTEN/MMAC1 gene may not play a major role in sporadic thyroid tumors.