Exercise intolerance and exertional dyspnea are common complaints in patients with sarcoidosis. Although in many cases these complaints are attributable to restrictive or obstructive lung mechanics or inefficiency of pulmonary gas exchange, other processes also may contribute to impairment in exercise function and may not be readily detected or distinguished from problems of lung mechanics on the basis of symptoms or routine laboratory testing. To identify the frequency and etiology of impaired exercise capacity in sarcoidosis patients with mild lung disease, integrative cardiopulmonary exercise testing was performed in 23 patients. Breath-by-breath measurements were made of gas exchange, ventilation, and heart rate. In 9 of 20 evaluable patients, the oxygen uptake (VO2) at the anaerobic threshold was low, and/or the rate of increase of VO2 was abnormal relative to work rate or heart rate, suggesting a defect in cardiocirculatory function. Resting and exercise echocardiography revealed normal left ventricular ejection fractions and wall motion in all nine of these patients, but findings suggestive of right ventricular hypertrophy and/or right ventricular dysfunction were present in five. Abnormal responses of VO2 during exercise are common in patients with sarcoidosis and may be due to subclinical impairment of right-sided cardiac function.