We herein present an autopsy case of occult lung adenocarcinoma that masqueraded as rapid progression of preexisting asbestosis. A 66-year-old man with an initial diagnosis of asbestosis experienced progressive dyspnea for over four years, and eventually died of respiratory failure. A series of chest computed tomography scans showed progression of extensive fibrosis from the right lower lobe to the left lobes. An autopsy revealed that well-differentiated adenocarcinoma with prominent reactive fibrosis was diffusely distributed within the fibrotic lesions in addition to the underlying asbestosis, and the unusually rapid progression of asbestosis was attributed to the reactive fibrosis of occult lung adenocarcinoma.