Four patients, aged between 17 and 56 years, were found to have pulmonary histiocytosis X. Radiologically they had reticulo-nodular increases in density and cystic changes in the lungs, especially in the upper fields. Pneumothorax and bone lesions were common. Despite the employment of transbronchial lung biopsies, open lung biopsy was almost always necessary to confirm the diagnosis. Broncho-alveolar lavage in one patient revealed 28% lymphocytes: T-suppressor cells predominated over T-helper cells (49% and 45%, respectively). Cell distribution returned to normal after steroid treatment.