The aim of the study was to examine the factors which play a role in the occurrence of differentiated thyroid cancer as related to anaplastic thyroid cancer (ATC). A case–control study was performed during the period 1993 to 2005. The case group comprised of 126 patients with ATC. The control group comprised of 126 patients with papillary thyroid cancer. Cases were individually matched by gender and place of residence. ATC was significantly related to older age (odds ratio = 1.11, 95% confidence interval = 1.07–1.14) and goiter (odds ratio = 1.20, 95% confidence interval = 1.07–1.35).