Fifteen patients with asthma were randomized into a double-blind, placebo controlled, crossover trial evaluating whether clonidine increased airway resistance. After meeting entry criteria [demonstration of 20% improvement in any two of the following pulmonary function tests after bronchodilator therapy: FVC, FEV1, or forced expiratory flow over the mid-50% of the vital capacity(FEF(25-75)], patients were evaluated on two subsequent visits, receiving sequential doses of clonidine 0.1 mg (maximum cumulative dose 0.3 mg) or placebo in a random fashion. They returned to clinic after a 36-48 hour washout period at which time they received the opposite treatment. The end points were three doses of clonidine or placebo, decrease in blood pressure to < or = 90 mm mercury systolic, and/or < 70 mm mercury diastolic, or a 20% fall in FEV1. Airway reactivity was proved utilizing methacholine on each clinic visit, and blood pressures were monitored continuously during the study period. There were no significant differences in the provocative concentrations of methacholine inducing a 20% fall in FEV1 (PD20) between baseline (1.92), clonidine (1.10), and placebo (1.69). (Numbers in parentheses indicate PD20 values in cumulative dose units).