The relationship between breast cancer and thyroid diseases is controversial. Discrepant results have been reported in the literature. The incidences of autoimmune and nonautoimmune thyroid diseases were investigated in patients with breast cancer and age-matched control individuals without breast or thyroid disease. Methods: Clinical and ultrasound evaluation of thyroid gland, determination of serum thyroid hormone and antibody levels, and fine-needle aspiration of thyroid gland were performed in 150 breast cancer patients and 100 control individuals. Results: The mean values for anti-thyroid peroxidase antibodies were significantly higher in breast cancer patients than in control individuals (P = 0.030). The incidences of autoimmune and nonautoimmune thyroid diseases were higher in breast cancer patients than in control individuals (38% versus 17%, P = 0.001; 26% versus 9%, P = 0.001, respectively). Conclusion: Our results indicate an increased prevalence of autoimmune and nonautoimmune thyroid diseases in breast cancer patients. There is always a controversy on the relationship between breast cancer and thyroid disorders. As these tissues, i.e., breast and thyroid, originate embryologically from the same type of cells, hypothyroid/hyperthyroid females are more prone to develop benign or malignant breast tumors. The studies on breast cancer patients indicate increased thyroid disorders in breast cancer patients, most commonly Hashimoto's thyroiditis accounts to increased thyroid disorders in these patients. This is independent of hormonal receptor status of the patient. These findings suggest the usefulness of screening for thyroid disease in any patient with breast cancer.