The objective of the present study was to investigate the dose-dependent bronchodilator efficacy and duration of action of the newly developed antimuscarinic agent tiotropium bromide in patients with chronic obstructive pulmonary disease (COPD). In a randomized, double-blind, placebo-controlled, crossover design, patients inhaled single doses of 10-80 micrograms tiotropium bromide and placebo, formulated in lactose powder capsules. The washout period between test doses was 72 h. Thirty five patients were enrolled in the trial (32 males and 3 females; mean age 64 yrs). Baseline forced expiratory volume in one second (FEV1) (mean 1.34 L) was less than 65% of predicted and was < 70% of forced vital capacity (FVC). All subjects had a smoking history of more than 10 pack-years. The mean reversibility of FEV1 after inhalation of 40 micrograms ipratropium bromide was 28%. Pulmonary function testing was performed before and at regular time intervals for up to 32 h after test drug administration. Compared to placebo, tiotropium bromide produced significant improvements in FEV1, FVC, peak expiratory flow rate (PEFR) and forced mid-expiratory flow (FEF25-75%). The bronchodilator response was almost immediate; peak improvement in FEV1 was reached 1-4 h after test drug inhalation, and the duration of action extended to 32 h after the 20, 40 and 80 micrograms doses. A clear dose-response relationship was seen for peak FEV1 and for the average FEV1 over differing time periods during the 32 h observation period, 80 micrograms of test drug being superior to the 10 micrograms dose. Peak improvement in FEV1 ranged 19-26% of test-day baseline for tiotropium bromide doses compared to 16% for placebo. The large improvement for placebo is probably due to carry-over effect which was significant. After excluding carry-over effect, the peak response to placebo decreased to 11%, whilst for tiotropium bromide doses it ranged 20-25%; standard error for mean difference was about 4%. There was no evidence of systemic anticholinergic effects. In this population of patients with COPD, tiotropium bromide was found to be a safe and long-acting bronchodilator, demonstrating a clear dose-response relationship following single dose administration.