High intakes of some fat-soluble vitamin or trace metals have been associated with a decreased risk of cancer. A mechanism to help explain their anticancer action might be immunosuppression during deficiency or immuno-enhancement with high intakes. In vitro, retinol suppressed T-lymphocyte functions, whereas high dietary vitamin A enhanced macrophage functions. High intakes of vitamin E can enhance some anticancer, immune defenses. Selenium excess was not very suppressive of immune functions in vitro, but did retard tumor cell growth. Selenium and zinc deficiencies are associated with immunosuppression. Enhanced immune functions by high intakes of trace elements and vitamins provide a mechanism to explain in part the concomitant decreased cancer incidence.