We analyzed the FEV(1)/FEV(6) and FEV(1)/FVC results of 502 consecutive patients in the spirometric diagnosis of airway obstruction. We also examined the agreement between FEV(6) and FVC in the spirometric diagnosis of restriction. Technically acceptable test results were obtained from 337 subjects (67%). The sensitivity of FEV(1)/FEV(6) for diagnosing airway obstruction as defined by FEV(1)/ FVC was 95.0%; the specificity was 97.4%. When interpretations differed, the measured values were all close to the lower limits of the reference ranges. When analysis included +/- 100-ml variability in FEV(1) and FEV(6), the sensitivity increased to 99.5% and the specificity to 100%. The reproducibility of FEV(6) was superior to that of FVC. These results suggest that FEV(6) is an accurate, reliable alternative to FVC for diagnosing airway obstruction and that FEV(6) is reasonably comparable to FVC for the spirometric diagnosis of restriction. FEV(6) is more reproducible and less physically demanding for patients.