Although acute effects from exposure to SO2 have frequently been observed at low levels in acute exposure experimental studies, it was not known whether or not such effects occur among workers chronically exposed at ranges of 0.3 to 4 ppm of SO2. Measurements of FVC, FEV1, FEF50, FEF75 and FEF50-75, and closing volume were made before and after the workshift for copper smelter workers and controls. Sputum samples for cytological examination were collected. Mean FEV1 and FVC were significantly decreased during a day's work in the smelter. Significantly more smelter workers had a decrease in FEV1 and FEF50 during the day than did controls. More of the smelter workers felt “chest tightness.” No change in closing volumes was seen. Smelter workers tended to have a higher percentage of sputum samples with moderate and marked atypia than did controls but the difference was not statistically significant.