Helminthic infections have been shown to inhibit allergy skin-prick tests and to modify the course of asthma. We evaluated Dermatophagoides pteronyssinus-specific immune responses in patients with asthma by measuring levels of T helper 2 (Th2) cytokines in peripheral blood mononuclear cell (PBMC) cultures. PBMCs from Schistosoma mansoni-infected patients with asthma living in an area of polyhelminthic endemicity produced lower levels of interleukin (IL)-5 and IL-4 in response to D. pteronyssinus antigen (Ag) 1 than did PBMCs from helminth-free patients with asthma. In contrast, D. pteronyssinus Ag 1-specific production of IL-10 was higher in helminth-infected patients than in helminth-free patients. The addition of recombinant human IL-10 to D. pteronyssinus Ag 1-stimulated cultures of PBMCs from helminth-free patients led to downmodulation of production of IL-5. After helminth-infected patients with asthma received antihelminthic treatment, there was down-modulation of D. pteronyssinus Ag 1-specific production of IL-10 in vitro. S. mansoni-infected patients with asthma produce lower levels of Th2 cytokines than do helminth-free patients with asthma, and this modulation is likely done by IL-10.