Objective: To investigate the estrogen receptor (ER) expressions in papillary thyroid carcinoma (PTC) and Graves' disease and the significances. Methods: The ER expressions in thyroid tissues from 35 PTC patients and 41 Graves' disease patients as well as 30 normal thyroid tissues (control group) were determined by immunohistochemical staining. The ER expression rates among groups were compared and their relations with age and sex of the patients were analyzed. Results: The ER positive rate in Graves' disease group (9.76%, 4/41) was lower than that in control group (16.67%, 5/30), but the difference did not reach statistical significance (χ2=0.25, P>0.05), while the ER positive rate in PTC group (80.00%, 28/35) was higher than that of control group, and the difference reached statistical significance (χ2=25.92, P<0.05). The ER positive rate in Graves' disease group was not evidently related to sex (χ2=0.62, P>0.05). In PTC group, the ER positive rate in female patients was significantly higher than that in male patients (χ2=4.52, P<0.05), but it was not associated with age of the patients (χ2=1.35, P>0.05). Conclusion: ER expression is reduced in the thyroid tissue of Graves' disease, which may be probably one of the predisposing factors for this condition; ER expression is increased in PTC tissue, which may be one of promoting factors for the occurrence and development of this tumor.