A field study of 239 horses was conducted to determine the efficacy and safety of clenbuterol HCl, a beta 2-adrenergic bronchodilator, when administered incrementally to effect in the treatment of chronic obstructive pulmonary disease (COPD). The severity of COPD (heaves) and response to treatment was determined by clinical evaluation; an overall 'heaveiness rating' (OHR) was assigned at each observation. The horses were treated orally b.i.d. with clenbuterol (as Ventipulmin Syrup), beginning with the lowest dosage of 0.8 micrograms/kg. On day 10 of treatment at the effective dose (0.8, 1.6, 2.4 or 3.2 micrograms/kg), treatment was either withdrawn (Schedule A) or continued for an additional 20 days (Schedule B). Horses on Schedule A demonstrated a significant improvement in the mean OHR on treatment Day 10 compared to the baseline overall heaveiness rating (BOHR) and a significant increase in the mean OHR (relapse) after the drug was withdrawn. Schedule B horses showed significant improvement (compared to BOHR) on treatment Days 10, 20 and 30. Incremental dosing with clenbuterol provided clinical improvement in 75% of the horses with a lower 95% confidence limit of 71%. Twenty-five percent were nonresponders. A greater percentage of the more severely affected horses required the 3.2 micrograms/kg dosage or were nonresponders compared to horses with a lower BOHR. Side effects of sweating, muscle tremor, and nervousness were of low intensity (mild to moderate) and frequency (< 7% of all observations) due to the regimen of incremental dosing to effect.