Résumé : Benign and malignant thyroid tumors constitute a wide range of neoplasias showing recurrent chromosome abnormalities. Cytogenetic studies of thyroid hyperplasias and follicular adenomas revealed hyperdiploïd karyotypes with a characteristic sequence of trisomies (7, 5, 12, 14, 16, 17, 20 and 22) starting with trisomy 7. Comparative genomic hybridization (CGH) findings on thyroid oncocytic tumors showed similar chromosomal gains with no difference observed between adenomas and carcinomas. Follicular thyroid carcinomas exhibit losses of 3p25-pter predominantly or of 22,13 and 1p segments. Formation of fusion genes PAX8 - PPARgamma1 caused by a t(2;3)(q13;p25) has been observed in several cases of follicular carcinomas only. Loss of chromosome 22 has been found most frequently associated with widely invasive follicular carcinomas. Activation of the RET protooncogene through chromosome rearrangements involving subband 10q11.2 represent the most common and specific genetic alteration in papillary thyroid carcinoma. Several chimeric genes resulting in the fusion of the tyrosine kinase domain of RET with the 5' sequences of different genes have been described. Germline mutations in RET are associated with medullary thyroid carcinoma in multiple endocrine neoplasia type 2 (MEN2). Cytogenetics of thyroid tumors, using conventional and molecular methods (FISH, CGH) demonstrated that particular chromosome aberrations may be related to the clinical behavior of these tumors and may provide informations for their diagnosis or prognosis.