Anaplastic thyroid cancer is very serious disease with bad prognosis and unknown etiology. The aim of the study was to test some hypotheses about other factors in addition to goiter related to anaplastic thyroid cancer occurrence. A case-control study was performed during the period 1993-2005. The case group comprised 126 patients with newly diagnosed anaplastic thyroid cancer. The control group comprised 252 patients who had for the first time goiter operation, and had no malignancy of thyroid gland. Cases and controls were individually matched by age, sex and place of residence (urban/rural). According to conditional multivariate logistic regression analysis, anaplastic thyroid cancer was significantly related to lower education (odds ratio=1.85, 95% confidence interval=1.21-2.82), other malignant tumors in personal history (odds ratio=4.37, 95% confidence interval=1.11-17.31), blood group B (odds ratio=3.69, 95% confidence interval=1.10-12.49), menarche at >or=15 years of age (odds ratio=2.63, 95% confidence interval=1.15-5.88), and first full-term pregnancy before 19 years of age (odds ratio=2.96, 95% confidence interval=1.26-6.96). On the basis of the results obtained, risk factors for anaplastic thyroid cancer are similar to risk factors for differentiated thyroid cancers.