We investigated the ability of patients with AIDS to develop antibody responses to a naturally encountered antigenic stimulus, Giardia lamblia. Using an enzyme-linked immunosorbent assay to detect IgG, IgM, and IgA to G. lamblia trophozoites, we tested sera from 29 patients with AIDS (15 without and 14 with G. lamblia infection); 20 healthy homosexual men; and 91 immunocompetent heterosexual subjects, 25 of whom were infected with G. lamblia. Heterosexual subjects infected with G. lamblia had significantly higher levels of all three classes of specific antibody than did the uninfected subjects (P <.0001). Patients with AIDS who had acute symptomatic giardiasis had significantly lower levels of all antibodies than did the heterosexual subjects who had giardiasis; specific IgM was absent in all but one patient with AIDS. The symptomatically infected patients with AIDS had low levels of G. lamblia-specific antibodies that were similar to those of the uninfected patients with AIDS. Patients with AIDS do not have to suffer from prolonged symptomatic G. lamblia infections, however, because available therapy is effective against the parasite, independent of a patient’s immune status. © 1988 The University of Chicago.