Angiotensin converting enzyme (ACE) inhibitors produce a dry, nonproductive cough in some patients. Retrospective surveys have suggested an incidence of cough of between 0.7 and 14%. Those patients who develop cough show a marked increase in the sensitivity of the cough reflex to inhalation of the extract of red pepper, capsaicin. They have a normal response before treatment, and the sensitivity of the cough reflex returns to normal when therapy is discontinued. The mechanism of this important side effect is not known. Further studies are required to clarify the mechanism of cough, both as a side effect of therapy and as a common symptom of respiratory disease.