After tobacco, marijuana is the most widely smoked substance in our society. Studies conducted within the past 15 years in animals, isolated tissues, and humans indicate that marijuana smoke can injure the lungs. Habitual smoking of marijuana has been shown to be associated with chronic respiratory tract symptoms, an increased frequency of acute bronchitic episodes, extensive tracheobronchial epithelial disease, and abnormalities in the structure and function of alveolar macrophages, key cells in the lungs' immune defense system. In addition, the available evidence strongly suggests that regularly smoking marijuana may predispose to the development of cancer of the respiratory tract. "Crack" smoking has become increasingly prevalent in our society, especially among habitual smokers of marijuana. New evidence is emerging implicating smoked cocaine as a cause of acute respiratory tract symptoms, lung dysfunction, and, in some cases, serious, life-threatening acute lung injury. A strong physician message to users of marijuana, cocaine, or both concerning the harmful effects of these smoked substances on the lungs and other organs may persuade some of them, especially those with drug-related respiratory complications, to quit smoking.