We undertook this study to determine the prevalence of normal roentgenograms in chronic diffuse infiltrative lung diseases. Of 458 patients with such disorders histologically confirmed, 44, or 9.6 per cent, had normal pre-biopsy films. In this group with normal x-ray films, desquamative interstitial pneumonia, sarcoidosis and allergic alveolitis were the most frequent diagnoses. Dyspnea was the principal complaint, and fine rales were common. The vital capacity was reduced in 57 per cent, and the single-breath diffusing capacity in 71 per cent. In half, histological changes and functional impairment were moderately severe. Films may be normal in such cases because isolated foci are too small or too few, because diffuse interstitial or intra-alveolar disease may cast no discrete shadows or because the lesions primarily affect airways or blood vessels. Patients with normal chest roentgenograms and normal mechanics of breathing but with impaired gas exchange should have lung biopsy for early diagnosis and therapy.