Measurement of peak flow rate (PER) is useful to assess venilatory capacity of a critically ill patient and particulary in the evaluation of obstructive ventilatory disease because of it easy management. A number of studies have already shown the usefulness of PFR measurement for rapid determination of pumonary reserve, for preoperative assement of pulmonary function or in evaluating changes in patient's degree of pulmonary disability. In view of the close correlation between PFR & MBC(Maximal breathing capacity) peak flow determination should also be of value in patients in whom the MBC cannot be measured easily because of the patient's illness or lack of sustained cooperation. In this study, the correlation coefficient (r) was found to be 0.77 between PFR & MBC, 0.72 between PFR & FEV1(forced expiratory volume for one second) and 0.73 between PFR & VC(vital capacity). Therefore we think that the measurement of peak flow rate in detecting ventilatory disease or in following a patient's ventilatory capacity is recommended.