In a randomized, double-blind, crossover study consisting of three 1-month periods, we compared the effects of ipratropium bromide (IB) 120 micrograms delivered by a metered dose inhaler (MDI) against two different doses of IB (125 micrograms and 500 micrograms) delivered by a gas driven nebulizer. Thirty-two patients (mean age 57.1 years, range 17-78) with severe airflow obstruction (mean PEFR 192 litres/min, range 75-380 litres/min) were recruited to the study. They had a maximum documented reversibility of over 20% (mean 55.2, range 25-200), and an improvement of over 13% (mean 26.3, range 13-56) in the PEFR to a test dose of 120 micrograms IB delivered by a MDI. In the 20 patients who completed the trial no significant differences were found between the treatment periods when comparisons were made of the weekly averages for the morning PEFR, evening PEFR, reversibility, diurnal variation of the PEFR, bronchodilator and steroid usage. The monthly assessments of FEV1, FVC, VC, RV/TLC ratio and the single breath transfer factor showed all three treatments to be better than the base-line assessment (P less than 0.001), but overall there was no significant difference between treatments. The 6-minute walking distance did not show any improvement over the base-line values with any of the treatments. Symptom scores also showed no overall difference between treatments except for cough which was worse on the 500 micrograms nebulizer solution (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)