The authors followed changes in clinical course of asbestosis, comparing groups of individuals working in exposure to chrysotile-asbestos dust, patients having so-called late asbestosis and reference group (individuals with absent or minimal roentgenologic changes in lungs). Analysis of 530 case histories proved increase of average length of service in dusty conditions before asbestosis development, longer progression of pulmonary fibrosis. Asbestosis progression does not depend on medical resolutions. "Late" asbestosis occurs in retired workers who were engaged into auxiliary occupations. Low baseline values of respiratory function and associated respiratory infection worsen the overall prognosis.