Fibroblasts in healthy adult lung are quiescent, synthesizing little collagen. We studied lung biopsies from 30 patients with pulmonary fibrosis, using immunohistochemistry with monoclonal antibodies against the propeptides of type I collagen to localize fibroblasts actively synthesizing collagen. Adjacent sections were stained with antibodies to type III and IV collagen, fibrin, cytokeratin, plasma fibronectin, or EDIIIa-containing “cellular” fibronectin (cFN). In rapid pulmonary fibrosis, including the proliferative phase of diffuse alveolar damage, organizing pneumonia, and subacute idiopathic fibrosis, collagen-synthesizing cells were numerous in organizing exudate filling airspaces but were also seen in the interstitium of the alveolar walls, interlobular septa, and walls of blood vessels. The new matrix deposited in the airspaces also contained type III collagen and EDIIIa-containing fibronectin. In chronic pulmonary fibrosis, more than half of the biopsies showed foci of collagen synthesis and cFN deposition near the air-tissue interface. The foci were consistently localized outside remnants of basal lamina and therefore within airspaces. The results indicate that (1) fibrosis in chronic idiopathic pulmonary fibrosis results mainly from organization of exudate within airspaces, just as it does after acute lung injury, and (2) during this process, fibroblasts increase their synthesis of collagen and fibronectin coordinately. Foci of active matrix deposition provide evidence for the progressive nature of chronic pulmonary fibrosis.