BACKGROUND Previous studies have demonstrated a high prevalence of thyroperoxidase antibodies (TPOAb) and autoimmune hypothyroidism in breast cancer (BC). These studies have been performed in BC patients generally 20-30 days after mastectomy. It is known that stress may have an influence on the immune system and a relation between stressful events and the onset or worsening of autoimmune thyroid disorders has been reported by several authors. The aim of the study was to evaluate the prevalence of autoimmune thyroid disease in patients with nodular breast disease selected for surgery before any treatment. Our hypothesis was that the high prevalence of thyroid autoimmune disorders in BC is independent of stressful events represented by surgery and/or anaesthetic procedures. METHODS Our series included 61 consecutive women aged 52.8 +/- 10.2 yrs (mean age +/- s.d.) with nodular breast disease selected for breast surgery: 36 out of 61 of them (59%) had BC and 25 out of 61 had benign breast disease (BBD). Controls included 100 healthy age-matched women. All patients and control subjects were submitted to clinical, ultrasound thyroid evaluation and serum-free thyroxine (FT4), serum-free tri-iodothyronine (FT3), TSH, TPOAb and thyroglobulin antibodies (TgAb) determination. RESULTS Mean FT3, FT4 and TSH concentration showed no differences between BC patients, BBD patients and controls. The prevalence of TPOAb in BC patients (12/36: 33.33%) was significantly higher than in BBD patients (5/25: 20%) (P < 0.01) and in controls (8/100: 8%) (P < 0.01). Similarly, the prevalence of TgAb in BC patients was 12 out of 36 (33.33%) significantly higher than that detected in BBD patients (4/25: 16%) (P < 0.01) and in controls (12/100: 12%) (P < 0.01). Of the 36 BC patients, 20 showed a diffuse hypoechogenicity of the thyroid gland to ultrasound evaluation, significantly higher than in BBD (7/25: 28%) (P = 0.03). Of the 20 BC patients who showed a hypoechogenic pattern of thyroid gland, 10 (50%) were associated with antithyroid antibodies positivity (TAb). This finding was present in two of seven BBD (28.57%) (P < 0.0001). Only two controls showed focal hypoechogenicity of the thyroid gland. Generally, 24 out of 36 (66.7%) of BC and 9 out of 25 (36%) of BBD (P = 0.02) had signs of thyroid autoimmunity consistent with the hypoechogenic pattern of thyroid gland associated or not with TAb; 2 out of 36 (5.55%) of BC and 1 out of 25 (4%) of BBD patients had autoimmune hypothyroidism and no hypothyroidism was found in controls. CONCLUSIONS The results of this study confirm the strong relation between thyroid autoimmunity and BC. This finding is independent of stressful events represented by surgery or anaesthetic procedures. The present data call attention to the usefulness of screening for autoimmune thyroid disorders in patients with nodular breast disease selected for surgery.