The objective of this study was to determine the accuracy of a semiquantitative method of assessing the relative degree of cellularity and fibrosis compared with a morphometric analysis of specific histopathologic features in Idiopathic pulmonary fibrosis (IPF). Morphometric analysis was performed on biopsy tissue from 20 patients. Morphometry showed that approximately 70% of parenchymal tissue was abnormal: 35% cellular consolidation and fibrosis, 20% honeycomb changes, 10% thick alveolar septa, and 5% small airways within abnormal parenchymal tissue. The prominent components of the abnormal parenchymal tissue were extracellUlar fibers, an abundance of Interstitial cells, and epithelial cell hyperplasia. Correlation between four groupings of pathologic features, Identified by a semiquantitative analysis, and each of the components of the parenchymal lesions showad significant correlations between (1) the fibrotic or reparative factor (alveolar wall metaplasia, fibrosis, honeycombing, and smooth muscle and vascular changes) and components of honeycomb lesions (extracellular fibers, interstitial cells, and epithelial injury and repair), and (2) the Inflammatory and exudative factor and extracellular fibers in the lesions. These results support that the scoring system used by the pathology panel provides an accurate assessment of pathologic features useful in the assessment of the extent and severity of the histopathologic lesions of IPF. AM REV RESPIR DIS 1992; 146: 1042-1047