Autonomously functioning thyroid nodules are frequently associated with mutations of the thyrotropin receptor. We analyzed a part of exon 10 of the thyrotropin receptor gene (base pairs 1762-1976) by direct sequencing and found missense mutations in 5 of 14 cases (codons 629, 631, 632, 633). Histologically, 3 of the 14 nodules were adenomas whereas 11 were hyperplasias. Nodules with mutations did not show significant differences from nodules without mutations with respect to age, histology, size, additional (non-functional) nodules and clinical symptomatology. Our results confirm that thyrotropin receptor mutations are involved in the development of autonomously functioning thyroid nodules. In this context, the terms hyperplasia and neoplasia should be reevaluated.