Anaplastic carcinoma of the thyroid gland is one of the most malignant tumors. Recently, DNA ploidy measured by flow cytometry and image analysis has been suggested as an additional useful indicator of tumor behavior. Studies on the occurrence and clinical significance of DNA aneuploidy in anaplastic carcinoma of the thyroid are rare. In this study, the pattern of DNA ploidy was measured by image analysis on Papanicolaou stained slides in four cases of anaplastic carcinoma and also measured by flow cytometry using paraffin blocks in two cases. In all cases of anaplastic carcinoma, DNA aneuploidy was found by image analaysis. By flow cytometry, one case had a diploid peak and the other case had an aneuploid peak. According to the above results, we conclude that anaplastic carcinoma of the thyroid glands have a high incidence of DNA aneuploidy and image analysis using Papanicolaou stained slides is a useful method in detecting DNA aneuploidy.