A new spirometric parameter for lung function assessment, the effective time at the 60-70% part of the forced vital capacity of the lung (teffp7), was considered. A total of 79 subjects were studied in two groups, A and B. In group A (5 healthy non-smokers, 22 patients with obstructive lung disease and 22 with pulmonary fibrosis), forced exhalation and the Helium closed-circuit technique were performed. From these tests, a number of lung function parameters were calculated, including teffp7. In group B (30 patients with bronchial asthma), the same tests were carried out before and 15 min after the administration of two puffs of a bronchodiluent, a beta 2-stimulant. From these tests, various lung function measures were again calculated, as well as the change in these parameters after bronchodilatation. The study shows that teffp7 is a more sensitive index than all the other parameters considered for determining airways condition, either in healthy subjects or in those with airways obstruction. In patients with pulmonary fibrosis, the other parameters considered are also less sensitive than teffp7. From the study of the change after bronchodilatation in patients with bronchial asthma, change in teffp7 was greater than in any other parameter. As teffp7 was the more affected parameter, it was also the more sensitive index for diagnosis of airways obstruction. In conclusion, teffp7 is a sensitive index for the diagnosis of airways obstruction or lung restriction, and it is markedly changed after bronchodilatation.