Recently, nuclear DNA contents of various human tumors were studied, and DNA aneuploidy was thought to have prognostic significance in many kinds of malignant tumors. Although the same significance was reported in thyroid carcinoma, the anaplastic thyroid carcinoma which is one of the most aggressive tumor in human malignant neoplasms, not always shows DNA aneuploidy. Therefore, using 3 xenografts established from 3 patients with anaplastic thyroid carcinomas, we investigated their growth activity, DNA ploidy and chromosome abnormalities. Two of these xenografts grew relatively fast in nude mice showing diploid or near diploid state in a flow cytometric study, and also, showed many structural abnormalities in chromosome analysis by the G-banding technique. The remaining one xenograft showed slower growth and aneuploidy, and had extensive numerical but less structural variability in its chromosomal constitution. These results indicate that some tumors showed DNA diploidy and have structural chromosome abnormalities and also suggest that the prognostic value of quantitative DNA measurement is limited in such tumors.