The aim of the study was to compare the prevalence of autoimmune thyroid diseases in three groups of women (66 with breast cancer (CaB), 68 with colorectal cancer (CaC) and 49 without oncological diseases as a control group). Serum levels of thyroid-stimulating hormone (TSH), free thyroxin (fT4), antibodies to thyroglobulin (TGB-ab) and thyroperoxidase (TPO-ab) and tumor markers CEA, CA 15-3 and CA 19-9 were investigated in all subjects by using the chemiluminiscence method. In contrast to Graves' disease (no observed case), autoimmune thyroiditis was diagnosed in 24.2 % women with CaB (4.5 % euthyroid and 19.7 % with subclinical or overt hypothyroidism), compared to 16.7 % in women with CaC (2.0 % euthyroid and 14.7 % with subclinical or overt hypothyroidism) and 16.2 % controls (4.0 % euthyroid and 12.2 % with subclinical or overt hypothyroidism). Serum levels of TGB-ab were higher in the group with breast cancer as compared to those with colorectal cancer and the control group (medians: 35.80 vs. 31.75 vs. 27.70, p<0.001). Similarly, the percentage of positive TGB-ab and TPO-ab serum levels was higher in women with breast cancer as compared to those with colorectal cancer and the control group. The results of the study support the controversial theory that there is an increased prevalence of autoimmune thyroiditis in women with breast cancer.