Airway Mycoplasma pneumoniae infection may be associated with asthma pathophysiology. However, the direct effects of M pneumoniae infection on asthma remain unknown. Using a murine allergic asthma model, we evaluated the effects of the different timing of airway M pneumoniae infection on bronchial hyperresponsiveness (BHR), lung inflammation, and the protein levels of T helper (Th) type 1 cytokines (ie, interferon [IFN]-γ), and Th2 cytokines (ie, interleukin [IL]-4) in BAL fluid. When M pneumoniae infection was administered 3 days before the allergen sensitization and challenge, the infection reduced the BHR and inflammatory cell influx into the lung. This was accompanied by a significant induction of Th1 responses. The mean (± SEM) IFN-γ levels in the BAL fluid were increased in the infected mice (490 ± 65 pg/mL) compared with those of the control mice (341 ± 36 pg/mL; p = 0.047). The mean IL-4 levels in the BAL fluid were reduced in the infected mice (41 ± 10 vs 212 ± 87 pg/mL, respectively; p = 0.056). The IFN-γ/IL-4 ratio was significantly higher in the infected mice (16.9 ± 5.6 vs 6.8 ± 2.2, respectively; p = 0.04). Contrary to this, when M pneumoniae infection was administered 2 days after the allergen sensitization and challenge, the infection initially caused a temporary reduction of BHR at day 3 after introduction of the infection and then increased BHR, lung inflammation, and IL-4 levels between day 7 and day 21 after introduction of the infection. The IFN-γ/IL-4 ratio in the BAL fluid of infected mice was increased at day 3 and then decreased at day 7 after introduction of the infection. Our data suggested that M pneumoniae infection could modulate both physiologic and immunologic responses in the murine asthma model. Our animal models also may provide a new means with which to understand the role of infection in asthma pathogenesis and may give evidence for the asthma hygiene hypothesis.