The flow-volume (FV) loop was introduced by Hyatt and associates in 1958. Its potential as a means of assessing lung function has been discussed. There are, however, few reports of its use in clinical studies. The purpose of this study was to add to the data on normal subjects and to quantify the differences between normal subjects and patients with selected types of lung disease. The FV curve was found to be reasonably effective in distinguishing normal subjects from those with lung disease, but did not provide a clear separation between subjects with asthma and those with other obstructive disease.