Graves' disease (GD) is an organ-specific autoimmune thyroid disease; 25-50% of GD patients will develop Graves' ophthalmopathy (GO). The etiology of GD and GO may be multifactorial, but the immune response plays a central role. Many studies have reported that IL-21 has crucial roles in autoimmune diseases. We examined whether IL-21 is associated with the development of GD and GO. The serum concentration of IL-21 was tested in 40 primary GD patients, 42 treated GO patients and 24 healthy controls. Our data show that the serum level of IL-21 is associated with the development of GD. We also made an association study with the IL-21 gene polymorphisms rs4833837, rs907715 and rs13143866 in a comparison of 633 patients and 612 healthy controls from the Chinese population. This case-control association study demonstrated that rs907715 SNP is significantly associated with GD, while the rs13143866 A allele is significantly associated with GO. The haplotypes A-G-G and A-A-A were found at higher and lower frequencies, respectively, in GD patients, suggesting a protective role for A-A-A. However, there were no significant differences in the frequencies of these haplotypes between the GO patients and the control group. We found no association between IL-21 gene polymorphisms and the age of GD onset. We conclude that IL-21 is associated with GD and GO.