The immune response and prevalence of certain autoimmune diseases is higher in females during the reproductive years than in males. One possible explanation for these differences is that women have higher levels of estrogen. Leukocytes in the peripheral blood and within the tissues have been shown to express both subtypes of the estrogen receptor (ER), ERα and ERβ. However, many studies on the presence of ER in different leukocyte subtypes clearly contradict each other. The function of ERα and ERβ in leukocytes has not been completely understood and the major part of information available today comes from studies on ER knockout mice. Estrogen and ERs have been suggested to regulate hematopoiesis and the production of immunoglobulins in leukocytes and to modulate the inflammatory response. In this review information about the presence and function of ERα and ERβ in different leukocyte subtypes will be summarised