The 1969 Oregon spirometric predictive equations were evaluated by retesting 199 of the 988 original sample population after 15 years. The 1969 data were used to test for sample bias between the retested and not-retested groups. There was no significant difference in mean values for age, height, or test results except for a five-year age difference in men. Regression analysis of residuals and the differences between calculated and predicted values of annual decrements of FVC, FEV1, and FEF25-75% on age revealed no statistically significant age trend. Although residual means were statistically significant for FVC and FEV1 for men and FVC and FEF25-75% for women, the differences between calculated and predicted annual decrements were significant only for women in FEF25-75%. Although group performance was accurately predicted for most tests, test SDs and SEMs demonstrated considerable individual variation. Lower limits of normality are suggested to assist in evaluating previously-tested patients.