The increased incidence and severity of Clostridium difficile infection (CDI) in older adults (age, ≥65 years) corresponds with the emergence of the BI/NAP1 strain, making elucidation of the host immune response extremely important. We therefore infected germ-free C57BL/6 mice aged 7-14 months with a BI/NAP1 strain and monitored the mice for response. Infected mice were moribund 48-72 h after infection and developed gross and histological cecitis and colitis and elevated concentrations of keratinocyte chemoattractant, interleukin 1β, monocyte chemotactic protein 1, and granulocyte colony-stimulating factor and decreased levels of interferon γ, interleukin 12 p40, interleukin 12 p70, and interleukin 10 compared with controls. We conclude that aged, germ-free C57BL/6 mice are susceptible to fulminant CDI from a BI/NAP1 strain and represent a novel model to further elucidate the host immune response to acute CDI.