Changes in end-expiratory lung volume (EELV) during exercise in normal subjects and in patients with severe chronic obstructive lung disease have previously been examined. To date there are no studies that have examined the changes in EELV in patients with mild to moderate lung disease. We studied the changes in EELV during exercise in patients with cystic fibrosis (CF) with a wide range of pulmonary impairment to determine if changes in EELV were related to the severity of lung disease. Twenty-two patients with CF were studied (FEV1 17 to 112% of predicted) during progressive bicycle exercise, and changes in EELV were determined by repeat measures of inspiratory capacity. Changes in EELV at end exercise ranged from an increase of 0.67 L to a decrease of 0.61 L, and significant relationships were found between the changes in EELV and resting lung function (FEV1 percent predicted r = 0.79 and VR/TLC r = 0.58), indices of maximal expiratory flow (FEF50 r = -0.72 and FEF25-75 r = -0.71), and maximal work capacity (W-Max r = -0.76 and W-Max percent predicted r = -0.69). For subsequent analysis, patients were divided into two subgroups. Patients who were able to decrease EELV during exercise (Subgroup A) had significantly better resting lung function and SaO2 and significantly higher W-Max, peak oxygen consumption, and SaO2 at W-Max. Patients in Subgroup A also had a near normal ventilatory pattern during exercise. In contrast, the patients who increased EELV during exercise (Subgroup B) had severe lung disease (mean FEV1 29 +/- 4 percent predicted), limited work capacity, and desaturated during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)