Newcastle disease virus (NDV) infection in chickens differing in vitamin A status has been selected as a model to examine the interrelationship between marginal vitamin A deficiency and the severity of consequences of measles infection in humans. Day-old chickens with limited vitamin A reserves, the progeny of marginally vitamin A-deficient hens, were fed purified diets containing either marginal (120 retinol equivalents/kg diet, ad libitum) or adequate (1200 retinol equivalents/kg diet, ad libitum or pair-fed) levels of vitamin A for a period of 10 wk. At 4 wk of age, half of the chickens in each group were infected intraocularly with the lentogenic, i.e., mildly pathogenic, La Sota strain of NDV. Within 1 wk of infection, plasma retinol levels in the infected, marginally vitamin A-deficient chickens showed a significant and persistent decrease compared to their noninfected counterparts fed the same diet. Moreover, infection with NDV resulted in increased rates of morbidity in the marginally vitamin A-deficient chickens compared with nondeficient chickens. The results of this study indicate that pre-existing marginal vitamin A status increases the severity of disease following NDV infection, and that infection with NDV reduces marginal plasma vitamin A levels to levels which can be regarded as deficient.