Mild to moderately severe hypoxemia was documented in 19 of 21 mostly asymptomatic firemen who were exposed to dense smoke containing polyvinylchloride and other pulmonary irritants. The hypoxemia was transient, with nearly complete reversibility within 24 hours. Subsequent tests for pulmonary function one month later demonstrated that this transient hypoxemia was not related to previous underlying pulmonary disease. The potential for serious complications of inhalation of smoke, particularly smoke from burning plastics, is heightened by the frequent lack of symptoms associated with the type of exposure, despite the development of hemoglobinemia. Precautionary measures should include uninterrupted use of an effective breathing apparatus by firemen exposed to smoke and to noxious fumes and temporary oxygen supplementation, even in asymptomatic firemen, following exposure to dense smoke.