Most studies of lung histology in systemic sclerosis have been based on autopsy specimens and consequently emphasize end-stage fibrotic disease. Although occasional pathologic descriptions of open-lung biopsies have recognized the presence of inflammatory cells, suggesting a similarity to "lone" cryptogenic fibrosing alveolitis, the two conditions have never been formally compared. In this study we describe the morphologic features of 49 open-lung biopsies from 34 systemic sclerosis patients with interstitial lung disease, many of whom had their lung disease diagnosed at an early stage. None had pulmonary hypertension. Examination of lung tissue by light microscopy showed the earliest changes to include patchy lymphocyte and plasma cell infiltration of the alveolar walls, interstitial fibrosis, and increased macrophages but only occasional polymorphonuclear cells and lymphocytes in the alveolar spaces. Alveolitis was not observed without fibrosis. Comparison of 22 biopsies with a similar number from patients with lone cryptogenic fibrosing alveolitis, matched for age and sex, revealed no qualitative or quantitative differences, other than a higher prevalence of focal lymphoid hyperplasia (follicular bronchiolitis) in the systemic sclerosis patients than in the lone cryptogenic fibrosing alveolitis controls (23 and 5%, respectively). There was an inverse correlation between the extent of interstitial inflammation and patients' age (p less than 0.05), disease duration (p less than 0.05), and age at onset of systemic sclerosis (p less than 0.01). There was also an inverse correlation between DLCO and interstitial fibrosis (p less than 0.01) and loss of lung architecture (p less than 0.05). Ultrastructural studies of eight systemic sclerosis biopsies showed evidence of endothelial and epithelial injury together with interstitial edema and excess collagen deposition. Occasional mast cells were observed, often in close contact with interstitial fibroblasts, but there were no tubuloreticular structures or evidence of immune complexes.(ABSTRACT TRUNCATED AT 250 WORDS)