During an outbreak of diarrheal illness among residents of a trailer park in rural Vermont, 37 (30%) of 122 residents met the case definition of outbreak-related giardiasis. Convalescent-phase sera from 24 residents and 20 nonresident control subjects were tested by enzyme-linked immunosorbent assay for immunoglobulin G (IgG), IgM, and IgA antibodies to Giardia lamblia. Residents showed higher levels of parasite-specific antibody than did nonresident controls for IgG and IgA but not IgM. Nine residents with giardiasis had a higher median level of G. lamblia-specific IgA but not IgG or IgM than 15 healthy residents (0.61 versus 0.16 optical density units; P = 0.004). Moreover, parasite-specific IgA levels were higher in those consuming tap water than in those who did not (0.31 versus 0.08 optical density units; P = 0.03) and increased with increasing water consumption. Levels of serum antibody to G. lamblia, particularly IgA, may be useful in determining exposure to G. lamblia-contaminated water and illness from G. lamblia during waterborne outbreaks of diarrheal illness.