Interleukin-10–deficient mice develop a robust pulmonary inflammatory response but no airway hyperresponsiveness (AHR) to inhaled methacholine (MCh) following allergen sensitization and challenge. In the present study, we investigated the effect of respiratory syncytial virus (RSV) infection on AHR and pulmonary inflammation in allergic IL-10 − / − mice. Unlike littermate control mice, RSV-infected or ovalbumin (OVA)-sensitized/challenged IL-10 − / − mice failed to develop significant AHR. In contrast, sensitized/challenged IL-10 − / − mice infected with RSV did develop AHR accompanied by increased eosinophil numbers, both in bronchoalveolar lavage (BAL) and pulmonary tissue, and mucin production in airway epithelium. The cytokine profile in OVA-sensitized/challenged IL-10 − / − mice was skewed toward a Th1 response but after RSV infection, this response was more of a Th2 type, with increased IL-5 levels in the BAL. Studies with an RSV mutant that lacks the G and SH genes showed equal enhancement of the AHR response as the parental wild-type strain, indicating that G protein is not essential to this response. These data suggest that RSV infection can overcome the failure of development of AHR in allergic IL-10 − / − mice.