Because of unanswered questions about prediction equations for the single-breath carbon monoxide diffusing capacity (DLCO) and as part of a larger collaborative project, standardized DLCO measurements were carried out in a selected sample of 361 healthy nonsmoking volunteers (194 men and 167 women) living in the Barcelona metropolitan area (Spain). Except for the test FIO2 (0.18), the study essentially followed the American Thoracic Society (ATS) and European Community for Coal and Steel (ECCS) recommendations for standardizing the methodology of measuring DLCO. Prediction equations for ages 20 through 70 were calculated separately for both sexes. Simple linear equations using age, height, and body weight as independent variables predicted the DLCO indices (DLCO, VA, and DL/VA) as well as more complex equations. In addition, a complete analysis of the residuals (predicted measured values) showed that the assumptions of the multiple regression analysis (independence, homoscedasticity and Gaussian distribution of residuals) were fulfilled using simple linear equations. Correction for the instrumental and anatomic dead spaces decreased the DLCO an average of 4.7%. The standard error of estimates was lower than those reported from other series in the literature. The predicted values from this study were lower than those reported by some investigators and were in reasonable agreement with other studies. A portion but not all of the differences could be explained on the basis of recognized differences in testing methodology. The results of this study may be of value to clinical laboratories seeking predictive equations for DLCO most appropriate for their testing methodology and patient population, and may assist in the resolution of some controversies regarding differences among predictive equations for DLCO.