This paper is a review and interpretation of prior studies that have investigated the effects of hypoxia on immune function. A summary of current methods used to assess immune effector cell function in human beings is presented using in vitro models of mononuclear cell activation. Animal studies indicate that impaired host defenses against bacterial pathogens may be compromised, but that resistance to virus infections may remain intact. Data from human studies and animal models indicate that active immunization and B cell function are unimpaired, whereas T cell function is blunted following exposure to hypoxia. Mechanisms that may be responsible for alterations in normal immunoregulation are presented. The implications of altered immune function in subjects exposed to high altitude are discussed.