Benzodiazepine drugs have been shown to suppress respiratory function in patients with chronic obstructive pulmonary disease (COPD). We designed a placebo-controlled crossover study to compare the effects of a new benzodiazepine, estazolam ('ProSom'), with those of flurazepam ('Dalmane') on cardiopulmonary function in COPD patients. 29 patients completed all treatment phases (estazolam 2 mg, flurazepam 30 mg or placebo). Respiratory and cardiovascular function were assessed in awake patients on days 1 and 5 (acute and cumulative effects). Eight patients were also assessed during sleep in each period. The effects of estazolam and flurazepam on ventilatory response to CO2 and mouth occlusion pressure were no different from those of placebo. However, acute administration of flurazepam lowered tidal volume and increased inspiratory flow. Although no clinical signs of respiratory depression were observed with any long term treatment, flurazepam decreased oxygen saturation and inspiratory time and increased respiratory frequency. Neither drug altered breathing control during sleep. Our results indicate that estazolam 2 mg is equally as safe a hypnotic agent as flurazepam for patients with mild COPD.