5-Fluorouracil (5-FU) is an analog of pyrimidine and has been shown to display antitumor and immunomodulatory effects. However, the impacts of 5-FU in regulating asthma, an inflammatory disease associated with T helper cell 2 (Th2) responses, remain unclear. Here, we determine the modulatory effects of low-dose 5-FU on Th2 cell responses in asthma and delineate the underlying mechanisms using adoptive cell transfer and in vitro culture experiments. Our data show that low-dose 5-FU treatment not only inhibits the induction of asthma in allergen-sensitized mice but also abrogates the major features of asthma in mice with established disease. We find that this protection of 5-FU treatment against asthma is accompanied by a decrease in the number of lung monocyte-derived dendritic cells (moDCs) in the asthmatic murine. Furthermore, we show that adoptive transfer of moDCs reverses the inhibitory effects of 5-FU treatment on Th2 cell responses in asthmatic mice. Surprisingly, 5-FU treatment does not suppress surface maturation markers and immunogenicity of moDCs in the lungs of asthmatic mice. Instead, it induces apoptotic cell death of mouse moDCs both in vitro and in vivo. In addition to its impact on mouse moDCs, we observe that low-dose 5-FU treatment can induce apoptotic cell death of human moDCs derived from peripheral blood mononuclear cells in vitro. Together, our findings reveal that low-dose 5-FU ameliorates Th2 cell responses, which may be at least partially related to the induction of apoptotic cell death of moDCs in asthma.