Idiopathic bronchiolitis obliterans-organizing pneumonia (BOOP) is characterized by air space fibrosis of unknown origin. Clinical resolution under steroid treatment suggests the removal of the fibrotic lesion. Open lung biopsies of four patients with idiopathic BOOP were studied by immunochemistry and electron microscopy. Three distinct cell-matrix patterns of intra-alveolar bud were found to represent the sequential evolution of the fibrotic process: fibrinoid inflammatory cell clusters in which immunoglobulins and procoagulant factors (fibrinogen, factors VII and X) were identified; fibroinflammatory buds in which desmin-containing fibroblasts were observed migrating, proliferating, and secreting matrix proteins; fibrotic buds in which myofibroblasts organized a loose connective matrix predominantly composed of fibronectin and type III collagen. Extending forms of fibrotic buds may join contiguous alveoli. Fibrotic bud remodeling ability is correlated to the nature and organization of the matrix components but the factors permitting intra-alveolar matrix degradation must be characterized.