Patients with sarcoidosis are known to have histologic pulmonary abnormalities despite normal lung fields or conventional pulmonary function or both. These patients permit a useful assessment of the alleged greater sensitivity of the various measurements made during incremental cardiorespiratory exercise testing. Abnormal responses on such testing may provide insight into such complaints as dyspnea in these patients. Incremental exercise testing was performed on 30 patients with biopsy-proven sarcoidosis who had normal spirometry; 13 had clear lung fields radiographically. Of these patients, the 21 who had normal single-breath diffusing capacity for carbon monoxide (Dsb; [group A]) were compared with the 9 who had decreased Dsb (group B). Half of the group A patients had excessive ventilation and 38% had increased dead space to tidal volume ratio (Vd/Vt), but frequencies of these abnormalities were greater in group B, 89 and 78%, respectively. Ventilatory response, as minute ventilation to oxygen consumption ratios ventilatory equivalents, and deadspace to tidal volume ratio (Vd/Vt) ratios were higher in group B. Widened alveolar-arterial oxygen pressure differences were seen in 7 of 9 group B patients but only 1 of 17 group A patients. This study supports the clinical impression that occult pulmonary impairment may be present in patients (in this case, sarcoidosis patients) with normal pulmonary function, and corroborates the utility of exercise testing in demonstrating such impairment. Reduction in Dsb predicted greater frequency of abnormal exercise responses, especially in oxygenation.