Fifty-three subjects with positive skin prick test results to cat extract and rhinoconjunctival symptoms on exposure to cat dander were enrolled in a double-blind, placebo-controlled study of oral cat immunotherapy. Responses were assessed by development of symptoms and nasal blockage on exposure to an apartment contaminated with cat dander, by titrated skin prick tests, and by cat-specific IgG and IgE. A total cumulative dose of 2.5 x 10(6) allergy units or 436 U Fel d I were administered over a period of 3 months. Both groups of subjects had significantly fewer symptoms on exposure to cat dander during the course of the study, but there was no significant difference between active and placebo groups. There were no significant changes in either group in nasal blockage, skin prick test results, or specific IgG levels. Both groups had significant increases in cat-specific IgE, but there were no differences between groups. Subjects receiving active treatment had a slight excess of gastrointestinal complaints. Two subjects receiving active treatment experienced systemic symptoms: one had pulmonary edema, and the other had persistent asthma and urticaria, which may have represented reactions to the treatment. We conclude that oral cat immunotherapy with the preparation and doses used in this study is not effective.