A series of 14 thyroid carcinomas, characterized for their basal adenyl cyclase activity (ACA), was examined for the presence of activating point mutations in the TSH receptor (TSHR) gene. Sequencing of the carboxyl-part of this gene revealed the presence of a somatic and heterozygotic point mutation in codon 623 in three out of six tumors showing a constitutively enhanced ACA and a poor response to TSH stimulation. The mutation determines the substitution of a serine for an alanine in the third intracellular loop of the receptor, in a region critical for signal transduction. One tumor bearing a TSHR mutation presented also a N-ras point mutation. Both mutations were detected also in a lung metastasis of this tumor. Our data represent the first report of alterations in the TSHR gene in thyroid malign neoplasia. TSHR mutations may indeed participate, as well as the G alpha s protein (gsp oncogene), in the oncogenesis of some differentiated thyroid carcinomas presenting increased basal levels of cAMP and a poor response to TSH.