Poor subject cooperation during pulmonary function testing for disability evaluation can be detected by noting a reduction in maximum voluntary ventilation (MVV). The MVV was compared with the forced expiratory volume in one second (FEV1) from normal subjects and persons with airways obstructive diseases. The MVV was found to be related to the FEV1, and the MVV can be estimated by multiplying the FEV1 by 40. The actual MVV value can then be compared with the estimated value to assess subject cooperation. Since upper airway obstruction and restriction due to chest wall or neuromuscular disease can also reduce the actual MVV, subjects who have a decrease in MVV should have a more through evaluation of their ventilatory status.