Objectives: To investigate the prevalence of respiratory impairment and its relation to underground exposure among coal workders with simple pneumoconiosis in Taiwan. Methods: This study investigated 284 coal miners, including 122 with X-ray Category 0 and 166 with Categories 1, 2, and 3 (simple pneumoconiosis), from the medical clinics of a hospital. Spirometric measurement and tests for pulmonary diffusing capacity (DLco) were conducted on each subject. Patients with progressive massive fibrosis (PMF) were excluded. Occupational histories were obtained from questionnaire survey. Results: The prevalence of an abnormally low FEV1/FVC (forced expiratory volume in 1s/forced vital capacity) in subjects with simple pneumoconiosis was 42.6%(69/162). For FVC and FEV1, it was 4.9(8/162) and 10.5%(17/;62), respectively. The prevalence of abnormally low spirometric values increased with higher X-ray categories and longer underground exposure. The prevalence of an abnormal DLco was also lower than that of FEV1/FVC. Conclusions: The prevalence of lung functional impairment in simple pneumoconiosis varies with the parameter measured, and DLco determination may be helpful in case of normal spirometric measurement.