McCune-Albright syndrome (MAS) is defined by the triad of café-au-lait skin pigmentation, polyostotic fibrous dysplasia, and hyperfunctioning endocrinopathies, such as precocious puberty, hyperthyroidism, GH excess, and Cushing's syndrome. This disorder is caused by sporadic, postzygotic activating mutations in the GNAS1 gene, which codes for the G(s)alpha protein in the cAMP signaling cascade. Nodular and diffuse goiters (with and without hyperthyroidism), as well as benign thyroid nodules, have been reported in association with MAS. Herein we report two cases of thyroid carcinoma in patients with MAS. The first is a case of papillary thyroid cancer detected incidentally during a hemithyroidectomy for hyperthyroidism in a 14-yr-old girl. The second is one of a 41-yr-old woman with long-standing MAS and an enlarging thyroid nodule, which was diagnosed as a clear cell thyroid carcinoma, a rare variant of thyroid cancer. Molecular analysis revealed that foci of malignancy and adjacent areas of hyperplasia and some areas of normal thyroid harbored activating mutations of Arg(201) in the GNAS1 gene. These findings suggest that the infrequent development of thyroid carcinoma in MAS patients involves additional mutational or epigenetic events.