Pulmonary histiocytosis X is a granulomatous disorder of the lung of unknown cause. Patients with this disease often complain of cough, dyspnea on exertion, and, occasionally, chest pain from a pneumothorax or bone involvement. However, it is not unusual for patients to be asymptomatic with only a diffuse interstitial infiltrate on their chest radiograph. The course is unpredictable and can range from spontaneous remission to progressive respiratory insufficiency and death. No therapy, beyond symptomatic and supportive care, has been shown to be effective. Pulmonary histiocytosis X appears to be a variation of the more disseminated histiocytosis X disorders, such as Letterer-Siwe disease and the Hand-Schüller-Christian syndrome. All of the histiocytosis X disorders have a characteristic histiocyte on pathologic study, with a unique ultrastructural organelle. Recent work suggests that this histiocyte is similar to macrophages found in the skin and some lymphoid organs. The histiocytosis X disorders may represent an abnormal response of the mononuclear phagocyte system to an antigen, to an immunodeficiency state, or both.