Several studies have investigated allergic airway inflammation, a T helper 2 (Th2)-type immune response, using a mouse model of asthma. At present, however, no reports have described sex differences in the sensitivity of late airway inflammation (LAI). The LAI induced by ovalbumin in adult BALB/c mice was compared in males and females or sham-operated males and castrated males. The males showed less severe bronchial-bronchiolar inflammation with infiltration of eosinophils and lymphocytes and lower content of such cells in bronchoalveolar fluid than the females. Moreover, interleukin-4 (IL-4) mRNA expression levels in splenic cells were lower in the males than in the females. Castrated males performed like the females. Moreover, when compared with the sham-operated males, the castrated males showed lower testosterone levels in the blood. The present results suggest that less sensitivity for LAI in the males may be because of the decreased Th2 cell responses compared with the females. Moreover the testosterone, at least in part, may be responsible for the decreased Th2 cell responses in males in vivo.