Correlation between disease extent on computed tomographic (CT) scans and severity of clinical and functional impairment was obtained in 23 patients with usual interstitial pneumonia (UIP) by review of the clinical data, pulmonary function tests, chest radiographs, and CT scans. The CT scans and chest radiographs were each read twice by two independent observers. Disease extent was assessed on CT scans by a visual score (0%-100% involvement of the lung parenchyma) and on the radiograph by the International Labour Office classification. There was good intraobserver and interobserver agreement for both CT and radiograph scores (all r greater than or equal to .71). CT scans gave a better estimate of disease extent and showed more extensive honeycombing than did the radiograph. A significant correlation was found between the extent of disease as assessed with CT and the severity of dyspnea (r = .62, P less than .001), as well as between CT and impairment in gas exchange as assessed by the diffusing capacity (r = .64, P less than .001). There was poor correlation between disease severity as assessed with chest radiography and the clinical and functional variables (all r less than or equal to .39).