The pulmonary function of 113 copper smelter workers was measured in 1973 and again in 1974 to asses the effects of chronic exposure to sulfur dioxide. Respiratory symptoms, smoking history, occupational history, and pulmonary function as measured by forced vital capacity and 1-sec forced expiratory volume were recorded. Exposures to sulfur dioxide, respirable particulates, sulfates, and copper were sampled at intervals throughout the year by personal monitoring. Exposure to 1.0 to 2.5 ppm of sulfur dioxide was associated with excessive loss of 1-sec forced expiratory volume during the year and an increase in respiratory symptoms, after controlling for smoking. No significant interaction between sulfur dioxide and concurrent exposure to respirable particulates on pulmonary function was found. Workers with 1-sec forced expiratory volume below normal on initial measurements (based on their age and height) showed evidence of even greater losses of pulmonary function related to sulfur dioxide exposure.