In 21 of 100 consecutive subjects demonstrating greater than or equal to 10 percent increase in one second forced expired volume following 200 micrograms inhaled salbutamol, the forced expired flow over the middle half of the vital capacity (FEF25-75% was unchanged (n = 16) or fell greater than or equal to 10 percent (n = 5). Volume adjustment of FEF25-75%, to the same volume below total lung capacity (TLC) before and after administration of the bronchodilator resulted in significant increases in 18 of these 21 subjects. The volume-adjusted FEF25-75% increased more (98 percent) following 1500 micrograms inhaled metaproterenol in ten asthmatic subjects, and decreased more (53 percent) following inhalation of histamine in eight subjects, than did the standard FEF25-75% (44 percent increase and 34 percent decrease respectively). When the small changes in TLC which were seen following metaproterenol (4.4 +/- 4.1 percent reduction, p < 0.01) and histamine (11.0 +/- 7.2 percent increase, p < 0.001) were considered, and FEF25-75% volume adjusted so it was measured at the same absolute lung volume, greater changes were seen (198 percent increase after use of metaproterenol, and 75 percent decrease after use of histamine). When FEF25-75% is being examined following administration of a bronchodilator or a bronchoprovocation challenge, it should be volume-adjusted to absolute lung volume. When TLC cannot easily be determined repeatedly (eg routine pulmonary function studies), volume adjustment to the same volume below TLC is advised.