Patients with systemic sclerosis are known to have histologic pulmonary abnormalities despite normal chest radiograph or conventional pulmonary function or both. In an attempt to detect early features of lung involvement in progressive systemic sclerosis, we investigated patients with systemic sclerosis using cardiopulmonary exercise testing. We have studied 78 patients who fulfilled the American Rheumatism Association criteria for the classification of systemic sclerosis, and according to the classification of LeRoy, 44 had limited cutaneous systemic sclerosis and 34 had diffuse cutaneous systemic sclerosis. A significantly decreased diffusing capacity (65 +/- 3% of that predicted) was present only in the group with diffuse cutaneous systemic sclerosis. The patients with lung involvement showed a significant reduction in exercise capacity (54 +/- 3% of that predicted) and in oxygen uptake (70 +/- 3% of that predicted). Additionally, we could demonstrate an increased functional dead space ventilation (0.34 +/- 0.02) and widened alveolar-arterial oxygen difference during exercise (44 +/- 3 mm Hg). By cardiopulmonary exercise testing, 12 of the 78 patients (15%) with normal single-breath diffusing capacity for carbon monoxide had increased dead space to tidal volume ratio. Our results suggest that occult pulmonary impairment may be present in patients with normal pulmonary function and that cardiopulmonary exercise testing enables detection of such impairment. Our study results show the limitations of resting data in predicting abnormalities during exercise in patients with systemic sclerosis.