To assess the effect of chronic sleep deprivation on host defense, we observed growth and regression of a subdermal allogenic carcinoma (Walker 256 rat tumor) in rats undergoing 10 days of total sleep deprivation (TSD rats), yoked stimulus control (TSC) rats that were partially sleep deprived, and home cage control (HCC) rats. Tumor size was measured daily. Integrated tumor size was smaller in TSD rats than in both TSC (P = 0.04) and HCC rats (P = 0.0003). Thus host defense against these tumors (as defined by reduction in tumor size) was improved by sleep deprivation. This improvement could be a nonspecific effect, e.g., tumor growth can be inhibited by a catabolic state (dietary restriction). TSD and TSC rats lost body weight, indicating a catabolic state. However, tumor size was not predicted by body weight change, but was predicted by change in sleep time (P = 0.02). Host defense enhancement could alternatively result from enhanced immune response. Early tumor size (5 days) was similar in the three groups, but peaked sooner in TSD rats than in both TSC (P = 0.05) and HCC rats (P = 0.01), leading to large differences in size later. Immune-suppressed rats also showed little difference from HCC rats in early growth but large differences later. Thus host defense in an in vivo model that manifests a systemic immune response can be enhanced by sleep deprivation with timing, which is consistent with an enhancement of the immune response.