Epidemiological studies show a very high relative risk for first degree relatives of probands with thyroid cancer. The familial form of nonmedullary thyroid carcinoma (NMTC) gives a more severe phenotype and appears earlier than its sporadic counterpart. Moreover, benign thyroid pathologies are often observed in NMTC kindreds. Little is known about the genetic risk factors of the disease. To study them, an international consortium has been organized at the International Agency for Research on Cancer over the past 2 yr to collect biological samples from NMTC families. The only genes known to be directly involved in susceptibility to NMTC are MNG1 on chromosome 14q32 and TCO on chromosome 19q13.2, previously localized by us and others. In addition to those two genes, the genes for Cowden's syndrome and familial adenomatous polyposis are associated with thyroid cancer, but not as an indicative phenotype. Another important gene in thyroid carcinogenesis is RET, which is mutated in the majority of cases of hereditary medullary thyroid cancer and rearranged in an important fraction of sporadic cases of NMTC. Here we report the result of a linkage analysis performed on the 56 more informative kindreds we have collected through the international consortium. Linkage analysis using both parametric and nonparametric methods excluded MNG1, TCO, and RET as major genes of susceptibility to NMTC and demonstrated that this trait is characterized by genetic heterogeneity.