It is well documented that sex-dependent factors affect susceptibility to infection, with most mouse models demonstrating higher resistance in females. We made the unexpected observation that infection with the intracellular bacterium Listeria monocytogenes showed an opposite pattern in several commonly used inbred mouse strains: female C57BL/6J, BALB/c, C3H/HeN, and CBA/J mice were significantly more susceptible to Listeria infection. The pronounced sensitivity of females to Listeria, which was revealed by significantly higher lethality rates, correlated also with increased bacterial numbers in organ tissues (spleen and liver) and several immunological changes in peripheral blood samples. Surprisingly, increased severity of infection in females was associated with elevated interleukin-10 (IL-10) levels in plasma. Experiments using Il10 knockout mice, for which no differences between the susceptibilities of males and females to Listeria infection could be detected, confirmed the important role of this immunosuppressive cytokine for the outcome of disease. Our findings are likely to have clinical relevance, since similar sex differences with regard to infection with Listeria monocytogenes and other intracellular pathogens have been reported for humans.