There is renewed interest in the classification and methods of recording adventitious pulmonary sounds. This is a study of the importance of fine crackles (rales) in the diagnosis and estimation of the severity of diffuse infiltrative pulmonary disease. Among 272 cases documented by lung biopsy, bilateral fine crackles were heard in 60 percent of those with interstitial pneumonias and asbestosis but in only 20 percent of those with sarcoidosis and other granulomatoses. These incidences were identical in 335 patients diagnosed clinically. In 322 selected ambulatory patients with chronic obstructive pulmonary disease, fine crackles were recorded in only 10 to 12 percent, while coarse crackles were not uncommon in patients with chronic bronchitis. In workers exposed to asbestos, crackles correlated with exposure. In serial studies of such workers, the occurrence of crackles alone appeared to be a random event, while among those with crackles together with one other of five criteria, almost one-half developed asbestosis within four to six years. Fine crackles correlated with pathologic severity, with radiographic honeycombing, and with physiologic abnormalities.