To investigate the characteristics and clinical significance of respiratory function in patients with Parkinson's disease (PD), we studied 38 patients (male, 19; female, 19: mean age, 65.5 years: mean duration of disease, 6.7 years) who had no history of respiratory disease and smoking. Fifty three non-respiratory disease subjects (male, 26; female, 27: mean age, 67.6 years) were served as age-matched control. We measured spirometry and maximal expiratory flow-volume curve in all patients, and analyzed the relations between respiratory function variables and clinical profiles. The clinical disability of PD was indicated by Hoehn-Yahr (H-Y) scale. The number of PD patients was 15 in H-Y 2, 18 in H-Y 3 and 5 in H-Y 4, respectively. The values of % VC, %FEV 1, FEV 1/FVC, %PEFR, % V50 in H-Y 4 group were significantly smaller than those in H-Y 2 and 3 groups. Small airway dysfunction (SAD) was represented by abnormality of % V25, % V50/V25. The prevalence of impairment in % V25 and % V50/V25 was detected in 13 patients (34.2%) and 15 patients (39.5%), respectively, this was significantly higher than age-matched controls. However, the mean value and prevalence of impairment in % V25, % V50/V25 were not affected by H-Y scale and duration of disease or ideal body weight (%predicted value). Twenty seven patients showed normal ventilatory function based on % VC over 80% and FEV 1/FVC over 70%. The prevalence of impairment in % V25, % V50/V25 was detected in 8 patients (29.6%), 9 patients (33.3%), respectively, among 27 patients with normal ventilatory function. These results suggest that ventilatory dysfunction is concerned with clinical disability but SAD which is independent of clinical disability seen prevalently in patients with PD. It is widely accepted that patients with PD frequently have cardiac or bowel dysfunction based on the visceral autonomic dysfunction. We hypothesize that SAD may also be caused by possible autonomic dysfunction in patients with PD.