Rationale: Currently, spirometric detection of airway obstruction in adults requires separate predictive formulae for each ethnicity and sex for %FEV₁/FVC and %FEV₃/FVC, the major measurements for defining airway obstruction. Objectives: Eliminate the need for multiple formulae for black, Latin, and white men and women by developing single formulae with less variance than current formulae for %FEV₁/FVC and %FEV₃/FVC. Methods: Data from nearly 6,000 healthy never-smokers ages 20.0 to 79.9 years in the National Health and Nutrition Examination Survey-III were reevaluated mathematically and graphically, based on the preliminary hypothesis that predictive normal FEV₁/FVC and FEV₃/FVC ratios could be calculated from the age and FVC alone, without considering ethnicity, sex, or height. Current and new formulae were evaluated, first considering the population as consisting of 36 equally weighted subgroups (6 decades X 3 ethnicities X 2 sexes), and then weighting each individual equally. Measurements and Results: For each year of age the slope of %FEV₁/FVC versus FVC approximated -1.8%/L/year; the slope of %FEV₃/FVC versus FVC approximated -0.8%/L/year. After trial and error iterations, the optimal formulae were: %FEV₁/FVC = 98.8 - 0.25 X years - 1.79 X FVC and %FEV₃/FVC = 105.4 - 0.20 X years - 0.75 X FVC. Conclusions: These two new predicting formulae for % FEV₁/FVC and %FEV₃/FVC, which require only age and FVC as variables, approximate actual values closer than previously published separate formulae for each ethnicity and sex. With 95% confidence limits, they should allow better discrimination between normality and airway obstruction in adults of at least these three ethnicities.