The quaternary amine, benzalkonium chloride, has been associated with cutaneous and mucosal delayed hypersensitivity reactions and with paradoxical bronchoconstriction after use of aerosolized asthma medications. Until now, quaternary amines have not been reported to cause occupational asthma. We describe a case of occupational asthma caused by prolonged exposure to a cleaning solution containing benzalkonium chloride in the workplace. Single-blind, placebo-controlled, open-room challenges were performed to determine the specific agent responsible for the patient's symptoms. Pulmonary function and epicutaneous challenge tests were also performed. The patient had positive responses to challenges with a liquid toilet bowl cleaner containing benzalkonium chloride but was unreactive to other agents tested. Removal from the workplace resulted in complete resolution of symptoms. The exact mechanisms responsible for occupational asthma induced by quaternary amines remain unknown; however, this case emphasizes the importance of recognizing reactive chemicals as possible causes.