In cultured fibroblasts initiated from fresh tumor biopsies of 11 patients with thyroid tumors, 5 medullary carcinoma of the thyroid (MCT) and 6 papillary follicular, the chromosomes are relatively unstable when compared with those from control fibroblast cultures. Tumor-derived fibroblast samples showed a mean number of cells in metaphase with chromosome abnormalities at a frequency of 9.1% and a range of 6-13%, whereas 5 controls exhibited a mean frequency of 4% and a range of 3-5%. Our results indicate that genetic instability in thyroid cancer patients is present not only in peripheral blood cultures, as reported earlier, but in their fibroblast cultures as well.