Cowden syndrome (CS) is an autosomal dominant inherited syndrome characterised by hamartoma development in multiple organs and a risk of breast, thyroid and other cancers. The susceptibility gene for this syndrome was mapped to 10q22-23. Subsequently, germline mutations in PTEN, which encodes a dual specificity phosphatase, were found in individuals and families with CS. With the identification of the CS susceptibility gene, DNA-based predictive testing may be offered in theory. Somatic mutations in PTEN have been described in sporadic thyroid tumors, endometrial carcinomas, prostate carcinomas and glioblastoma multiforme. Although initial analyses suggest that the presence of somatic PTEN alterations appear to be associated with more advanced disease in carcinomas of the prostate and brain, this does not appear to be the case in epithelial thyroid tumors.