Evaluation of impairment caused by exposure to an occupational toxin can be complicated by additional exposure to other injurious agents. Because cigarette smoking is common and cigarettes are implicated in obstructive lung disease and cardiovascular diseases, we assessed the contribution of smoking to functional abnormalities in a group of asbestos-exposed shipyard workers. Seventy-three workers who never smoked were paired with 73 current smokers by age and asbestos exposure. Pulmonary function and performance during cycle incremental exercise were compared between the 2 groups. Nonsmokers had significantly higher VC, FEV1, FEV1/VC, and diffusing capacity for carbon monoxide than did smokers. Only 3 of the 73 nonsmokers but 23 of the 73 smokers had a FEV1/VC below the 95% confidence limit of predicted value. The FEF25-75%, on the other hand, failed to identify additional subjects with obstruction not found by the FEV1/VC. During exercise, despite no difference in maximal heart rate, the maximal O2 uptake (VO2max) and oxygen-pulse were lower among smokers. In addition, smokers more frequently had abnormal AaPO2 at maximal exercise. Of 33 smokers who had a VO2max less than 80% of predicted, 16 were judged to have cardiac disease, whereas only 2 appeared to be limited by obstruction. Only 15 of the 73 nonsmokers had a VO2max less than 80%. We conclude that cigarette smoking was the major contributing factor to the obstructive lung disease observed in asbestos workers, and it also had a strong influence on the occurrence, nature, and magnitude of exercise limitation. The history of cigarette smoking has an important effect on the assessment of impairment from asbestos.