Morbidity and mortality are usually caused by respiratory disorders in Parkinson's disease (PD) because of pulmonary functional impairments. The purpose of this study was to determine the effects of PD on ventilatory function and that the use of pulmonary function tests (PFT) may serve as an indicator of PD severity. PFT have been performed in 21 patients with PD (15 non-smoker and six exsmoker with 36.17 +/- 26.54 pack-years of smoking history: mean age 64.67 +/- 10.76 years) and 16 normal age-matched control subjects who never smoked. The clinical disability was indicated by a Hoehn-Yahr (H-Y) scale. MEF25% [maximal flow rate at 25% of remaining forced vital capacity (FVC)] and FEV1 (the volume of air expired during the first second of the FVC) in exsmoker PD group was lower than non-smoker PD group (P < 0.05). The two effort dependent variables peak expiratory flow (PEF) and the maximal flow rate at 75% of the remaining FVC (MEF75%) percent predicted values were 70.66 +/- 24.15 and 69.05 +/- 24.39 in non-smoker PD group whereas 90.18 +/- 17.24 and 90.00 +/- 18.97% predicted were in control group. respectively (P < 0.05). The maximal voluntary ventilation (MVV) was found to be 52.83 +/- 15.52 and 91.52 +/- 13.80% in PD and control group, respectively (P < 0.0001). MVV was the most effected parameter that was inversely correlated with the PD severity (r = -0.87- P < 0.0001). We concluded that less coordinated and less explosive muscle force has contributed to decrease in PEF and MEF75% values. and MVV decreases in PD as a efficiency during repetitive motor tasks which in part reflects abnormal agonist-antagonist muscle activity. So, spirometric studies may serve as a useful indicator of patients' neurophysiological conditions for the purpose of anticipating and preventing complications because of pulmonary impairment.