The role of host immunity in cancer clinical outcomes has been wellestablished in animal models. In humans, the impact of the immune system as a therapeutic maneuver to treat malignancies has been proven by the development of graft-versus-tumor effect observed in the allogeneic stem cell transplantation. However, with few notable exceptions, no definitive conclusions have been reached as to the role of host immunity in humans and its impact on cancer outcomes. This article reviews the clinical evidence of how human immunity can affect cancer clinical outcomes using the absolute lymphocyte count as a surrogate maker of host immune competence.