We compared locally derived prediction equations for the FVC and FEV1 in Hispanics with external prediction equations derived from non-Hispanic whites. Algebraic subtraction of the external non-Hispanic equations from the New Mexico Hispanic equations showed that the differences would vary with age and height. The study population included 442 patients with Hispanic surnames between the ages of 25 and 80 yr evaluated at the University of New Mexico Hospital pulmonary function laboratory. We calculated percent predicted values for FVC and FEV1 using spirometric prediction equations from a New Mexico Hispanic population and from non-Hispanic white populations in Salt Lake City, Tucson, and six other U.S. cities. We employed either 80% of predicted or the lower fifth percentile as the criterion for separating normal from abnormal values. We found that the concordance of the classifications by the external and internal equations varied among the external equations and with the criterion used for abnormality. The classification of the FVC and FEV1 as normal or abnormal was influenced by the regression equation in about 5 to 10% of the subjects. Subjects with discordant classifications were not clearly predicted by age or height, although they tended to be at the extremes of the age and height distributions. This study shows that classification of lung function by locally derived ethnic-group-specific prediction equations may lead to a differing clinical categorization for some persons in comparison with external non-ethnic-group-specific equations.