Pulmonary hypertension (PHT) occurred in 59 (9%) of 673 systemic sclerosis patients seen between 1963 and 1983. In 30 patients, all with the CREST syndrome (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasias), the pulmonary hypertension was isolated, i.e., independent of other pulmonary or cardiac conditions. In 20 patients, isolated PHT was demonstrated by cardiac catheterization. All had normal or only mildly decreased lung volumes, and mild or no pulmonary interstitial fibrosis on chest roentgenogram. In comparison with 287 CREST syndrome patients without PHT, these 20 patients had markedly reduced diffusing capacity for carbon monoxide (DLCO) (mean 39% of predicted normal). In 6 patients, the low DLCO antedated clinical evidence of PHT by 1-6 years. At autopsy there was marked intimal fibrosis with hyalinization and smooth muscle hypertrophy in the small- and medium-sized arteries, without significant parenchymal fibrosis or inflammation. Patients with isolated PHT did not respond favorably to vasodilators and had a very poor prognosis, with a 2-year cumulative survival rate of 40%. A DLCO less than 45% of predicted in the absence of pulmonary interstitial fibrosis may be an important predictor of the subsequent development of isolated PHT.