The aim of the study was to compare the prevalence of autoimmune thyroid diseases (AITD) in patients with breast and colorectal cancer and controls and to evaluate the impact of AITD on the outcome of patients with breast cancer. Serum levels of TSH (thyroid-stimulating hormone), FT4 (free thyroxine), TPOAb (antibodies to thyroid peroxidase), TgAb (antibodies to thyroglobulin), selenium and prolactin were investigated in 210 randomly chosen women (89 with breast cancer and 72 with colorectal cancer after breast or abdominal surgery and 49 controls without oncological diseases). Eighty-four women with breast cancer were followed for a median of 136.0 months. The prevalence of positive titres of TPOAb (>60 kIU.l(-1)) was higher in the women with breast cancer as compared to positive titres in women with colorectal cancer and the controls (29.8 vs. 12.5 and 12.2%, respectively, P=0.016 and 0.036, respectively). Similarly, the prevalence of clinical, ultrasound and laboratory documented AITD was higher in women with breast cancer as compared to that in women with colorectal cancer and the controls (35.7 vs. 18.1 and 16.3%, respectively, P=0.014 and 0.029, respectively). We did not find any prognostic significance of FT4, TSH, TgAb, TPOAb, prolactin and the presence of AITD on relapse-free and overall survival among women with breast cancer. A negative prognostic significance of body mass index and serum levels of selenium on relapse-free survival was found. In conclusion, the prevalence of euthyroid AITD was higher in women with breast cancer as compared to euthyroid AITD in women with colorectal cancer and controls. The presence of AITD did not have an impact on the outcome of women with breast cancer.