1. We investigated the effect of 2 weeks therapy with oral cetirizine (10 mg twice daily) on methacholine bronchial reactivity in 14 asthmatics. All had bronchial hyperreactivity to methacholine on entry to the study, with a geometric mean cumulative PC20 of 0.83 mg ml-1 (range 0.1-3.61 mg ml-1) and eight were atopic on skin prick testing. 2. The study was placebo-controlled and in randomised order, with a minimum 1 month washout period between cetirizine and placebo study periods. Four methacholine challenges were performed in all, at the beginning and at the end of each study period. 3. No significant change in methacholine reactivity was found following 2 weeks therapy with cetrizine. The repeatability coefficient for the PC20 methacholine over 2 weeks was found to be 2.90 doubling dilutions for the placebo period and 1.73 doubling dilutions for the cetirizine period. The study had a 80% power at the 5% significance level (two-tailed) to detect a 1.16-doubling concentration change in methacholine reactivity. 4. We conclude that 2 weeks therapy with cetirizine has no significant effect on non-specific bronchial hyperreactivity.