We examined the effects of ambroxol on spontaneous or stimulated generation of reactive oxygen species (ROS) by bronchoalveolar lavage (BAL) cells prepared from 6 patients with chronic obstructive pulmonary disease (COPD) and age-matched control subjects without COPD. The ROS produced by BAL cells were measured by the lucigenin-dependent chemiluminescence method. The application of ambroxol into culture media containing BAL cells inhibited spontaneous and stimulated generation of ROS by BAL cells harvested from COPD patients and control subjects in an ambroxol concentration-dependent manner. These results indicate that ambroxol may be a candidate agent for reducing oxidant stresses of airways in COPD.