Forty-nine patients with progressive systemic sclerosis who had undergone extensive studies including pulmonary artery catheterization as part of an ongoing prospective study of the natural course of progressive systemic sclerosis were evaluated. The overall prevalence of pulmonary arterial hypertension in this population of patients with progressive systemic sclerosis was 33 percent, and among 10 subjects with the CREST syndrome the prevalence of pulmonary hypertension was 50 percent. The relation between pulmonary arterial hypertension documented at catheterization and abnormal results of noninvasive studies suggesting pulmonary hypertension, including physical examination, chest x-ray, electrocardiography, echocardiography, single-breath diffusing capacity, and vital capacity, was studied. Diffusing capacity was significantly lower in those patients with definite pulmonary hypertension (mean pulmonary artery pressure of 22 mg Hg or more) compared with those with a normal mean pulmonary artery pressure, and a diffusing capacity below 43 percent of predicted showed the greatest sensitivity (67 percent) of any single diagnostic test in detecting definite pulmonary hypertension. Chest x-ray suggesting pulmonary hypertension was the least sensitive of the tests evaluated, but showed the greatest specificity (100 percent) in identifying patients with pulmonary hypertension. A classification matrix based on discriminant function analysis utilizing the combination of diffusing capacity below 43 percent of predicted and chest x-ray and electrocardiographic findings correctly identified 75 percent of patients with definite pulmonary hypertension and 97 percent of patients with a normal pulmonary artery pressure, but failed to identify correctly patients with mild pulmonary hypertension (mean pulmonary artery pressure of 20 mm Hg). These findings indicate that specific noninvasive studies are helpful in assessing the likelihood of normal or definitely elevated pulmonary artery pressures in patients with progressive systemic sclerosis, but patients with mild pulmonary hypertension are not likely to be identified by these noninvasive studies.