To define the cytokine response to Ascaris lumbricoides infection, the cellular immune response to adult and larval-stage Ascaris antigens in young adults with moderate infection intensities (n = 73) was compared with that of a group of uninfected control subjects (n = 40). A. lumbricoides-infected subjects had significantly greater lymphoproliferative responses to adult and larval-stage antigens, compared with uninfected control subjects (P < .01). The frequencies of parasite antigen—stimulated peripheral blood mononuclear cell (PBMC)-expressing interleukin (IL)—4 and IL-5 were significantly greater in the infected group (P < .001), whereas the frequencies of IL-10— and interferon-γ—expressing PBMC were similar in the 2 groups studied. The ratios of Th2 to Th1 cytokine frequencies were significantly elevated in the infected group, compared with those in uninfected subjects, as was IL-5 protein production by PBMC stimulated with adult (P < .05) and L3/L4 stage (P < .001) antigens. Analysis of these data indicates that A. lumbricoides infections in endemic regions are associated with a highly polarized type 2 cytokine response.