This report is based on 43 cases where a diagnosis of either bronchiolitis obliterans with organizing pneumonia (BOOP), usual interstitial pneumonia (UIP), or small airways disease (SAD) was established by lung biopsy. The severity of histologic abnormalities in the peripheral airways and interstitial spaces were measured on these biopsies using semiquantitative techniques and compared with the clinical data available in 42 of 43 cases, preoperative chest radiographs in 31 of 43, and preoperative pulmonary function tests in 29 of 43. The data show that when a diagnosis of BOOP was made, there was a higher total pathologic score for membraneous bronchiolitis (MB) and respiratory bronchiolitis (RB) than for UIP and SAD (p less than 0.005). This was due to peribronchiolar inflammation and the presence of loose connective tissue in the RB lumen. The pathologic changes in the interstitial space were less severe in SAD than in BOOP or UIP (p less than 0.005). Clubbing was more frequent in UIP (p less than 0.01), and symptoms were of shorter duration in BOOP (p less than 0.05). The radiographic assessment showed that the characteristic finding in BOOP was patchy air-space consolidation, a finding that was not present in UIP or SAD.