Susceptibility to pathogens and genetic variation in disease resistance is assumed to persist in nature because of the high costs of immunity. Within immunity there are different kinds of costs. Costs of immunological deployment, the costs of mounting an immune response, are measured as a change in fitness following immunological challenge. Maintenance costs of immunity are associated with investments of resources into the infrastructure of an immune system and keeping the system at a given level of readiness in the absence of infection. To demonstrate the costs of immunological maintenance in the absence of infection is considered more difficult. In the present study we examined the maintenance costs of the immune system in lines of Drosophila melanogaster that differed in their antibacterial innate immune response under starved and non-starved conditions. Immunodeficient mutant flies that have to invest less in the immunological maintenance were found to live longer under starvation than wild type flies, whereas the opposite was found when food was provided ad libitum. Our study provides evidence for the physiological cost of immunological maintenance and highlights the importance of environmental variation in the study of evolutionary trade-offs.