Short-term cultures of 19 follicular thyroid carcinomas were examined cytogenetically. Clonal chromosomal changes were detected in 12 tumors. Two follicular carcinomas had only numerical alterations: one with a hyperdiploid karyotype with trisomies/polysomies of chromosomes 7 and 12, similar to the karyotypes previously identified in a sub-group of benign thyroid lesions, and the other with monosomy 20. In the remaining ten cases several structural chromosome anomalies were found. Loss of the short arm of chromosome 3 was observed in one tumor. In two widely invasive and metastasizing follicular carcinomas there was a t(7;8)(p15;q24) as the sole abnormality in one case and a der(8)t(7;8)(p15;q24) together with other cytogenetic alterations in the other case. This finding suggests that t(7;8)(p15;q24) may be related to an aggressive behavior of follicular thyroid carcinomas. Genes Chromosomes Cancer 21:250–255, 1998. © 1998 Wiley-Liss, Inc.