Findings on the benefits and mechanism of action of vitamin A in measles and other infectious diseases and immunocompromised states are discussed. Vitamin A deficiency is one of the world's major malnutrition problems and is most commonly found in children under the age of five years. An association between vitamin A status and immune function has been suggested by community studies and animal experiments. Mortality and susceptibility to infection and diarrhea are higher in children with vitamin A deficiency. The association between increased mortality and morbidity and vitamin A deficiency is strongest in children with measles. Vitamin A supplementation reduces mortality and complications resulting from measles. Measles may increase the body's utilization of vitamin A, possibly because of the rapid destruction of epithelial surfaces. Vitamin A may boost immune responses in the elderly, persons with high exposure to ultraviolet light, patients who have undergone surgery, and persons with parasitic infection, but more studies are needed. The immune defect caused by vitamin A deficiency may be due to alterations in the glycoproteins of the lymphocyte membrane, an adverse effect on helper T-cell function, the effect on epithelial tissue, or some other mechanism. Vitamin A therapy is relatively safe, and its effectiveness in children with measles and possibly other groups appears to justify public health campaigns to eliminate vitamin A deficiency. Vitamin A apparently has important immunomodulating properties, notably in patients with measles.