The course of pulmonary asbestosis and its determinants have been examined in 280 applicants for compensation among former workers of the crocidolite mine and mill at Wittenoom Gorge, Western Australia. Serial chest radiographs accrued over more than 3 decades were graded for parenchymal disease separately by two observers according to the 1980 ILO Classification of Radiographs for Pneumoconioses and without knowledge of exposure histories or compensation details. In 136 subjects whose median duration of exposure was 37 months, radiographic asbestosis appeared between 1 and 34 yr after initial exposure and then progressed continuously. Total exposure to asbestos and time from first exposure to the appearance of definite radiographic asbestosis were significant determinants of the rate of progression of profusion of radiographic abnormality. Asbestosis should be considered to be an active disease even 3 decades after exposure has ended.