The advent of molecular biologic techniques has resulted in the recognition of bidirectional traffic of cells at the maternal-fetal interface. In this light, and because women are preferentially affected by a wide variety of autoimmune diseases, the subject of pregnancy immunology is of special interest. That pregnancy often induces remission of rheumatoid arthritis is an intriguing biologic observation for which further understanding may yield insights into disease pathogenesis. Another important question is the effect of pregnancy on susceptibility to autoimmune diseases. This question is highlighted by the recent finding that chimeric cells can persist for many years after pregnancy completion. At present, the biologic significance of microchimerism from pregnancy is an area of research that is for the most part unexplored.