Objective: To investigate the extent of asbestos exposure among patients with primary lung cancer in Japan. Methods: A retrospective estimation of potential asbestos-exposed individuals, as determined by the presence of pleural plaques identified on chest computed tomography (CT), was conducted on 885 pathologically confirmed primary lung cancer patients (mean age 71.3 years, 641 males). All patients were diagnosed at 29 hospitals across Japan between 2006 and 2007. Since these hospitals belong to the Japan Federation of Democratic Medical Institutions (MIN-IREN), an organization of medical institutions for workers, the study subjects may contain a higher proportion of workers than the general population. Results: Pleural plaques were identified in 12.8% of subjects (15.8% in males and 4.9% in females), consisting exclusively of cases older than 50 years. They were found most frequently on the chest wall pleura (96.5%), followed by the diaphragm (23.9%) and mediastinum (9.7%). Calcifications were seen in 47 cases (41.6%). The highest prevalence of pleural plaques was seen among workers from construction-related fields (37.7%). No distinct lung cancer histology was observed in patients with pleural plaques. Coexistence of pleural plaques and small irregular opacities was observed in 2.5% of subjects. Conclusion: In a Japanese population representing more workers than general Japanese, 12.8% of patients with primary lung cancer may have experienced asbestos exposure at some time in the past. Special medical attention should be paid to individuals with a history of employment in construction-related occupations, as workers in this sector showed the highest prevalence of pleural plaques.