Postpartum thyroid dysfunction is rather a common problem during postpartum period, found in approximately 5% of mothers in the general population. It occurs from subclinical autoimmune thyroiditis that is aggravated after parturition and causes various types of thyroid dysfunction. Immune activity is physiologically suppressed during pregnancy so that the fetus is not rejected, and rebounds above the normal level after parturition. Graves' disease and Hashimoto's thyroiditis spontaneously ameliorate during pregnancy, and are often aggravated after parturition. The high-risk mothers for postpartum thyroid dysfunction can be well screened by anti-thyroid microsomal antibody (MCAb) and 60% to 70% of MCAb-positive mothers develop postpartum thyroid dysfunction, which is transient in most cases. New onset of Graves' disease may be screened by thyroid stimulating antibody (TSAb) and 70% of TSAb-positive mothers develop either transient or persistent postpartum Graves' disease that usually occurs at 3-6 months postpartum. Immune rebound after parturition may not cause only autoimmune thyroid diseases but other autoimmune Table 4. Various types of postpartum autoimmune disease diseases, which may be investigated with the similar strategies to those in postpartum autoimmune thyroid disease. Thus, we found that postpartum onset of rheumatoid arthritis occured in 0.08% of women in the general population and could be partially predicted by measuring rheumatoid factors in early pregnancy. There are several case reports of other autoimmune diseases developed after delivery; postpartum renal failure or post-delivery hemolyticuremic syndrome, postpartum idiopathic polymyositls, postpartum syndrome with antiphospholipid antibodies, postpartum autoimmune myocarditis. Many other possible postpartum autoimmune diseases are still unexplored. Postpartum autoimmune thyroid syndrome is a good model for understanding the mechanism of onset and aggravation of other autoimmune diseases. Prediction and prevention of postpartum onset of many autoimmune diseases could be established in the near future.