Background: Dairy products have often been implicated as a cause of exacerbation of asthma, but there is little scientific evidence to support this hypothesis. Objective: We sought to determine whether dairy products induce bronchoconstriction in a group of adults with asthma. Methods: Twenty subjects with asthma (13 women and 7 men) were recruited from respondents who had previously completed a food and asthma questionnaire. Ten subjects perceived that their asthma became worse with ingestion of dairy products (positive perceivers), whereas ten were negative perceivers. None of the subjects had positive skin prick test results with cow's milk. The study was a randomized, cross-over, double-blind, placebo-controlled trial. Subjects complied with a dairy-free diet throughout the study. The active challenge was a single-dose drink equivalent to 300 ml of cow's milk. A positive reaction was defined as a 15% reduction in both FEV₁ and peak expiratory flow (PEF) on the active challenge day compared with results obtained at the same time on the placebo day. Results: For both FEV₁ and PEF there were no statistically significantly differences in group means between active challenge and placebo challenge, between sequence of administration, or between perceptions. Nine subjects showed FEV₁ or PEF changes that were greater than 15% of baseline values: four patients showed changes after both active and placebo treatment; two after treatment with placebo only; and three after active treatment alone. Of the latter group, two subjects showed changes only in PEF, and when one of these subjects underwent a further detailed study, no asthmatic reaction could be demonstrated. Conclusion: It is unlikely that dairy products have a specific bronchoconstrictor effect in most patients with asthma, regardless of their perception. (J Allergy Clin Immunol 1998;101:45-50.)