The effects of food availability on immune function of gray red-backed voles (Myodes rufocanus) exposed to low temperature (5°C) were examined under a short photoperiod (10L:14D). Twenty-seven voles, caught in the wild, were evenly divided into 3 groups adjusted for sample size, body mass, and sex ratio. Among the 3 food-manipulation groups (food control, food addition, and food restriction), the food-added group displayed enhanced immune response. In contrast, the food-restricted group displayed weakened immune response and the lowest body mass. Kidneys, hearts, and livers of food-restricted voles were significantly hypertrophied in comparison with food-control voles and food-added voles. Therefore, food limitation (quality and quantity) may cause a reduction in immune function and body mass at low temperature on short photoperiods because of maximum expenditure of acquired resources for thermoregulation and maintenance of body condition and, consequently, may induce higher mortality during cold and long winters.