We studied the prevalence of spirometric changes among asbestos insulation workers to investigate when functional abnormalities appear during the course of asbestos employment and the influence of cigarette smoking. Of 1,249 eligible asbestos insulation workers in the New York-New Jersey metropolitan area, 1,117 (89.4%) were examined in the year 1963 to provide baseline pulmonary function status for long-term prospective observation. Forced vital capacity (FVC) was measured in all 1,117 workers and forced expiratory volume in 1 second (FEV1) in 613 workers (55%). Of 353 workers examined in the first 10 years after onset of exposure, 26 (7.4%) had FVC below 70% of predicted, a prevalence similar to that reported in nonexposed general populations. Prevalence increased with time from onset of exposure. Of the 117 workers examined 40 or more years after onset of exposure, 76 (55%) had FVC below 70% of predicted. A similar trend with time was shown for FEV1 and FEV1/FVC. Cigarette smoking had little influence on the prevalence of pure restrictive impairment. Cigarette smokers and non-cigarette smokers had much the same prevalence (28%) of moderate to severe reduction of FVC while the FEV1/FVC was normal. None of the non-cigarette smokers and five of the cigarette smokers had a predominantly obstructive pattern. One non-cigarette smoker and eight cigarette smokers showed reduction of both FVC and FEV1/FVC, consistent with a mixed ventilatory abnormality. The data demonstrate that asbestos alone without the additional effect of cigarette smoking has no measureable effect on the function of the large airways.