Airway obstruction was measured by spirometry in 296 boilermakers with 15 or more years shipyard exposure to asbestos. Percent of predicted was used to adjust each worker's pulmonary function values for height (mean 174 cm), age (mean 52.5 years) and cigarette smoking (mean 23.3 years). Mean values were significantly (P less than 0.05) below predicted for FVC 4.23 l 94.2% FEV1 3.06 l 89.3%, FEF25-75 2.51 l sec-1 82.3% and FEF75-85 0.574 l sec-1 77.8%. Corresponding values for the 106 men with pulmonary asbestosis (ILO profusion of opacities 1/0 or greater), were below these levels. Those without radiographic signs of asbestosis had intermediate values significantly below predicted. Correlation coefficients for pulmonary functions with ILO categories of asbestosis (profusion of irregular opacities) were: FVC -0.2381, FEV1 -0.2494, FEF25-75 -0.2403 and FEF75-85 -0.1629. All were significant P less than 0.05. The subgroup with radiographic asbestosis (ILO 1/0 or greater), had more functional loss. Data on this large cohort of asbestos-exposed workers establish that airway obstruction occurs after 15 years of exposure and before the slightest profusion of asbestosis scarring in the lungs of shipyard workers, and worsens progressively with greater profusions of irregular opacities.