Polysomies of chromosomes 7 and 12 have been frequently observed by conventional cytogenetics in a subgroup of thyroid follicular adenomas and in some cases of thyroid goiters. To further study possible cytogenetic similarities between these two types of thyroid lesions, we have used fluorescence in situ hybridization (FISH) to detect polysomies of chromosomes 7 and/or 12 in isolated nuclei from frozen and paraffin-embedded material of goiters and thyroid follicular adenomas and compared results with previous ones obtained by flow cytometry and conventional cytogenetics. With a set of two alpha-satellite DNA probes specific for the centromeric regions of chromosomes 7 and 12, used either separately (single-target fluorescence in situ hybridization) or simultaneously (double-target fluorescence in situ hybridization), we detected polysomies of chromosome 7 in 35.7% of the thyroid follicular adenomas and in 10.7% of the goiters. Polysomies of chromosome 12 were detected in 29.6% of the thyroid follicular adenomas and 6.7% of the goiters. The significantly higher frequency of adenomas with numerical alterations for chromosomes 7 and/or 12 supports the idea of a biological continuum and karyotypic evolution between both lesions. It is also noteworthy that polysomies of chromosomes 7 and/or 12 were observed only in lesions with an exclusive (or predominant) microfollicular histological component, as detected by enzymatic in situ hybridization on frozen sections.